Metallurgical Investigation of VM Motori Exhaust Valves Vm...2.2 Applications The VM Motori model R-428 diesel engine was developed to be an affordable diesel engine for the general

1

Colorado School of Mines

Metallurgical Investigation of VM Motori Exhaust Valves

MME Senior Design Spring/Fall 2017 Rachel English, Erin Hamand, Anna Banks, Leo Martinez, Brian

Medberry, Adam Polizzi

Submitted to Dr. Gerald Bourne

12 December 2017

2

EXECUTIVE SUMMARY

This report presents a metallurgical evaluation of VM Motori exhaust valves to determine if the

company has responded to reports of valve failure, and to investigate possible mechanisms and

root causes of eventual engine failure. The new and used valves were tested and characterized

using SEM imaging, EDS elemental analysis, optical microscopy, carbon analysis, and

mechanical testing. The alloy determined to be used, 21-4N valve steel, is a commonly used

diesel exhaust valve alloy. Stress concentrations due to wear rings were found on the in-service

exhaust valves. This provides reasoning for a shortened fatigue life, which was calculated to be

approximately 300 million cycles to failure. It was discovered that there were no metallurgical

differences between new and used valves indicating that VM Motori has not responded by altering

the valve alloy or manufacturing process.

Rachel English Erin Hamand

3

TABLE OF CONTENTS Executive Summary ...................................................................................................................... 2 List of Figures ............................................................................................................................... 4 List of Tables ................................................................................................................................. 5 1.0 Introduction ......................................................................................................................... 6 2.0 Background ........................................................................................................................ 6

2.1 The Engine .......................................................................................................................... 6 2.2 Applications ......................................................................................................................... 7

3.0 Scope ................................................................................................................................. 7 4.0 Experimental Procedures ................................................................................................... 8

4.1 Visual Inspection ................................................................................................................. 8 4.2 Cleaning .............................................................................................................................. 8 4.3 Nondestructive Testing ........................................................................................................ 8 4.4 Metallographic Preparation ................................................................................................. 9 4.5 Etching and Optical Microscopy .......................................................................................... 9 4.6 SEM/EDS………………………………………………………………………………………….10 4.7 Carbon Content…………………………………………………………………………..………10 4.8 Vickers Hardness…………………………………………………………………………………10 4.9 Tensile Testing .................................................................................................................. 10

5.0 Results and Discussion .................................................................................................... 11 5.1 Visual Inspection ............................................................................................................... 11 5.2 Non Destructive Test ......................................................................................................... 11 5.3 Optical Microscopy ............................................................................................................ 12 5.4 SEM/EDS .......................................................................................................................... 13 5.5 Carbon Content…………………………………………………………………………………..14 5.6 Vickers Hardness .............................................................................................................. 14 5.7 Tensile Testing .................................................................................................................. 15

6.0 Conclusion ........................................................................................................................ 15 Appendix A: Introduction/Background Figures ............................................................................ 17 Appendix B: Experimental Procedures Figures………………………………………………………18 Appendix C: Visual Inspection Figures and Tables .................................................................... 19 Appendix D: Non-Destructive Testing Figures ............................................................................ 22 Appendix E: SEM/EDS Figures and Table ................................................................................. 23 Appendix F: Optical Microscopy Figures and Tables .................................................................. 26 Appendix G: Vickers Hardness Testing Figures and Tables ...................................................... 41 Appendix H: Tensile Testing Figures .......................................................................................... 43 References .................................................................................................................................. 46

4

LIST OF FIGURES Figure 1: Depiction of the underside of an engine head, which has been removed from the block, where a valve head has sheared off and fallen into the cylinder…………………………...17 Figure 2: Image of the VM Motori R-428 DOHC diesel engine[2] ............................................... 17 Figure 3: Tensile sample TN-1 mounted in flat grips at the stem, on bottom, and a fabricated top mount that cradled the head of the valve .................................................................................... 18 Figure 4: Macrograph of fracture surface showing indications of high-cycle fatigue including ratchet marks around diameter and beach marks. ..................................................................... 19 Figure 5: Macrograph of fracture surface showing severe damage from catastrophic failure. ... 19 Figure 6: U-1 after cleaning showing wear rings. ........................................................................ 20 Figure 7: U-2 after cleaning showing wear rings. ........................................................................ 20 Figure 8: U-3 after cleaning showing wear rings. ........................................................................ 20 Figure 9: Schematic illustration showing locations where diameters are measured. .................. 21 Figure 10: U-1 magnaflux results showing no strong indications of surface cracks. ................... 22 Figure 11: U-2 magnaflux results showing no indications of surface cracks. .............................. 22 Figure 12: Compositional BSE image of interface between head material (above) and stem material (below). ......................................................................................................................... 23 Figure 13: B-2 head material showing the presence of niobium carbides. ................................. 23 Figure 14: EDS spectra of carbides confirming composition as niobium carbides. .................... 23 Figure 15: Compositional BSE of N-1 head material showing niobium carbides. ....................... 24 Figure 16: Compositional BSE image of B-1 stem material. ....................................................... 24 Figure 17: EDS spectrum of B-1 stem material. ......................................................................... 25 Figure 18: B-1 Center 20X Magnification. ................................................................................... 26 Figure 19: B-1 Center 50X Magnification. ................................................................................... 26 Figure 20: B-2 Center 20X Magnification. ................................................................................... 27 Figure 21: B-2 Center 50X Magnification. ................................................................................... 27 Figure 22: B-2 Edge 20X Magnification ...................................................................................... 28 Figure 23: B-2 Edge 50X Magnification. ..................................................................................... 28 Figure 24: B-3 Center 20X Magnification. ................................................................................... 29 Figure 25: B-3 Center 50X Magnification. ................................................................................... 29 Figure 26: B-3 Edge 20X Magnification. ..................................................................................... 30 Figure 27: B-3 Edge 50X Magnification. ..................................................................................... 30 Figure 28: B-4 Center 20X Magnification. ................................................................................... 31 Figure 29: B-4 Center 50X Magnification. ................................................................................... 31 Figure 30: N-1 Center 20X Magnification. ................................................................................... 32 Figure 31: N-1 Center 50X Magnification. ................................................................................... 32 Figure 32: N-1 Edge 20X Magnification. ..................................................................................... 33

5

Figure 33: N-1 Edge 50X Magnification. ..................................................................................... 33 Figure 34: N-2 Center 20X Magnification. ................................................................................... 34 Figure 35: N-2 Center 50X Magnification. ................................................................................... 34 Figure 36: U-1 Center 20X Magnification. ................................................................................... 35 Figure 37: U-1 Center 50X Magnification. ................................................................................... 35 Figure 38: U-2 Center 20X Magnification. ................................................................................... 36 Figure 39: U-2 Center 50XMagnification. ....................................................................................36

Figure 40: U-2 Edge 20X Magnification.......................................................................................37

Figure 41: U-2 Edge 50X Magnification.......................................................................................37

Figure 42: U-4 Center 20X Magnification.....................................................................................38

Figure 43: U-4 Center 50X Magnification.....................................................................................38

Figure 44: B-2 Interface 20X Magnification…………………………………………………………..40

Figure 45: Vickers Microhardness B-1………………………………………………………………..42

Figure 46: Vickers Microhardness B-2………………………………………………………………..42

Figure 47: TN-1 Stress Strain Curves…………………………………………………………………43

Figure 48: TU-1 Stress Strain Curves…………………………………………………………………43

Figure 49: TN-1 and TU-1 Stress Strain Curves……………………………………………………..44

Figure 50: TN-1 Fracture Surface……………………………………………………………………..44

Figure 51: TU-2 Fracture Surface……………………………………………………………………..45

LIST OF TABLES Table 1: Primary automotive applications of the VM Motori R-428 DOHC engine........................7 Table 2: Grinding and Polishing method for samples ………………………………………………..9 Table 3: Recipe for Vilella's Reagent etchant used to etch the surfaces of optical microscopy samples…………….......................................................................................................................9 Table 4: Dimensional analysis along length of stem in used valves as compared to a new exhaust valve shows evidence of material loss during the service life of the valve.....................21 Table 5: Semi-quantitative EDS results for the bulk material composition of specimen B-2 head material........................................................................................................................................24 Table 6: Composition for 21-4N valve steel. ...............................................................................24 Table 7: Semi-quantitative EDS results for the bulk material composition of specimen B-2 head material........................................................................................................................................25 Table 8: Table of average grain size diameter from various locations in the austenitic phase, taken from the 20X magnification micrographs above.................................................................39 Table 9: Table of Carbon Content values for all samples…………………………………………..14 Table 10: Vickers’s hardness values for each specimen with an overall average for each specimen as well as the standard deviation................................................................................41

6

1.0 INTRODUCTION

There have been an increasing number of reports of exhaust valve failure in the VM

Motori R-428 diesel engine, which has raised concerns about the engine’s safety and reliability.

This engine has been manufactured from 2001-2013 and a significant amount of vehicle on the

road utilize this engine. The purpose of this investigation is to identify the possible causes of

these failures and to determine if VM Motori has responded to reports of engine failure in any

way by changing the design or altering the manufacturing process of the valves. These exhaust

valves have been known to fail as a result of the valve head separating from the stem.

After the valves fracture, the broken components fall into the cylinder chambers inside

of the engine block and cause further engine damage. Failure of this nature is likely to happen

near 150,000 miles, or 300 million cycles, and prolonged operation of the vehicle following

these failures can ultimately lead to catastrophic engine failure. The damages caused by these

broken valves can require extensive repairs or full replacement of the engine, which can place

a heavy financial burden on the owner. In addition to the high costs of engine repair and

replacement, these failures pose a risk to the operator’s safety.

2.0 BACKGROUND 2.1 The Engine

Diesel engines such as the R-428 dual overhead camshaft are known for their reliability

and long service lifetimes which can generally last for of hundreds of thousands of miles with

regular maintenance. These engines typically require minimal repair which makes them an

attractive option for many consumers. The VM motor, depicted in Figure 2 of Appendix A, is a 4-

cylinder diesel engine where the cylinders are arranged linearly, as opposed to the V-type

geometry or the opposing flat array that is used in many other diesel engines. This engine type

features 4 valves per cylinder and is the first family of VM engines that utilizes the common rail

injection system, which designates the way that the fuel is injected into the cylinders. This is

7

done by a collective rail that ensures each cylinder gets the same fuel injection pressure and

therefore the same volume of fuel during operation [1].

The purpose of the exhaust valves is to allow the exhaust gases and pressure produced

by the fuel combustion to exit the cylinder into the exhaust pipe and into the open environment.

This is done to prevent overheating during combustion and it also serves to withhold the escape

of fuel and air pressure during the compression cycle of the cylinder when closed.

2.2 Applications

The VM Motori model R-428 diesel engine was developed to be an affordable diesel

engine for the general automotive consumer. Table 1 shows a recent history of automotive

applications. This investigation of the exhaust valves is of importance because many vehicles

may be at risk of engine failure as these models become older and reach higher mileages.

Table 1: Primary automotive applications of the VM Motori R-428 DOHC engine Year Auto Manufacturer Model

2001-2007 Chrysler Voyager

2001-2004 (Europe Only) Jeep Cherokee/Liberty

2005-2007 Jeep Cherokee/Liberty

2004-Present BMC Megastar

2012-2013 Chevrolet Colorado

3.0 SCOPE

Failure of exhaust valves and mitigation of failures by VM Motori was the focus of this

investigation. The scope included comparing new valves to valves taken out of service both

broken and unbroken. With support from Dr. Gerald Bourne at the Colorado School of Mines

and Jim Hulse, our outside contact, and an auto mechanic with many years of experience

working with the VM Motori R-428, our team obtained numerous failed valves, used valves

removed from service before failure, and commercially available new valves. These valves

8

were initially an assortment of intake and exhaust valves. The intake valves used in this engine

have no history of premature failure and were visually inspected but not tested.

All used valves examined had been removed from engines where a valve had failed, but

due to the limited number of valves that were available for testing, a statistical analysis was not

completed in comparison to the general number of valves in circulation. This work provides

insight into the effects of the service conditions on the metallurgy and properties of the valves

and proposes any possible causes that may have contributed to the exhaust valve failures.

4.0 EXPERIMENTAL PROCEDURES 4.1 Visual Inspection

A visual inspection was performed on the as received condition of the valves including

used and fractured as well as used unfractured. New valves were obtained and a visual

inspection was performed on them as well. Macroscopic images of the used fractured valves

as well as the used unfractured valves are presented in Appendix 3. Attention was given to

fracture surfaces, deformation, surface condition, and any other signs of damage that may

indicate the source of failure. Findings were noted.

4.2 Cleaning

The valves were cleaned using acetone and the remaining carbon deposit was

removed by mounting the valves in a drill press and using a 240 grit sandpaper to lightly scrub

the surface, with care being taken not to gouge the metal.

4.3 Nondestructive Testing

After as-received inspection, nondestructive testing was conducted in the form of

magnaflux dye penetrant to locate any micro-cracks in the surface of the valves. Two dyes were

utilized in the magnaflux testing red dye penetrant and Zyglo® penetrant. Red dye makes

cracks visually apparent with the naked eye while the Zyglo® penetrant requires an ultraviolet

9

(UV) light to reveal surface flaws. Pictures were taken from the procedure and findings were

noted.

4.4 Metallographic Preparation

Valves from each of the three conditions, used unbroken, used broken and new, were

sectioned longitudinally across the failure region and mounted in Bakelite to be imaged using

optical microscopy and scanning electron microscopy (SEM). Location of the sample within the

valve was noted for each sample to determine position of weld from the bottom of the sample.

Each sample was ground and polished using the method shown in Table 2

Table 2: Grinding and Polishing method for sample N-1, U-1 through U-4, and B-1 through B-4.

Grind or Polish Abrasive (grit or μm for

polish) Time (mins) Grind 120 n/a

Grind 240 n/a

Grind 320 n/a

Grind 600 n/a

Grind 1200 n/a

Polish 6μm n/a

Polish 1μm n/a

Vibramat 0.5 μm 120

4.5 Etching and Optical Microscopy

Before SEM and EDS, the polished mounted samples were etched using Vilella’s Reagent of

the composition shown in Table 3 [3].

Table 3: Vilella's Reagent etchant used to prepare all optical microscopy samples

Vilella’s Reagent

Picric Acid 2.5g

Hydrochloric Acid 12.5mL

Ethanol 250mL

10

Optical microscopy was performed on etched samples to reveal the grain structure and

identify patterns or differences between the specimens. The three concentric circle method

was employed to determine grain size in the bulk and on the edges of the head material. The

location of the weld from the bottom of the valve was measured after etching.

4.6 SEM/EDS

SEM images of two broken, one used and unbroken and two new samples were taken at

100X and 1000X magnification. Energy dispersive spectroscopy (EDS) data was also collected

for elemental analysis collected for elemental analysis and for bulk material near the head as

well as precipitates. The elemental analysis is used to determine the material type. The results

of the analysis are located in Appendix E.

4.7 Carbon Content

Using LECO Carbon/Sulfur Analytical Equipment, the carbon content of all three sample

conditions were determined. Two samples were tested for new and used condition and one

sample was tested for broken.

4.8 Vickers Hardness

Vickers hardness testing was performed according to ASTM E-92 on all samples using

the auto-Vickers hardness testing machine and manually testing [4]. All sample types were

tested with a linear pattern of ten hardness indentations. The values for each sample condition

were compared to each other.

4.9 Tensile Testing

Tensile tests were performed on three different samples, new valve TN-1 and used valves

TU-1, and TU-2. ASTM E-8 was used for these tests with modifications to accommodate the

geometry of the valves [5]. The valve required slight grinding on opposite sides of the head to fit

11

inside the cradle mount, which can be seen in Figure 3 of Appendix B. Also, the stem of the valve

was ground so it would have more surface area where the flat tensile grips contacted it in an

attempt to ensure that the valve would not slip through the grips when tested.

5.0 RESULTS AND DISCUSSION

5.1 Visual Inspection

Fracture surfaces of the failed valves were examined. Of the four valves that were

obtained which had failed in service, three were too damaged to analyze. All failure occurred in

the stem close to the valve head. The relatively well-preserved fracture surface is depicted in

Figure 5 of Appendix C. Ratchet marks around the diameter indicate multiple fatigue cracks

initiating at the surface. Presence of beach marks and a small overload region indicate a

relatively low cyclic stress contributing to failure.

A representative example of the failed specimens that were too damaged to analyze is

shown in Figure 6. Dimensional changes were observed across the length of the stem in the

used valves, as shown in Figures 7-9. Measurements are shown in Table 4 and a schematic

representation of the locations measured is in Figure 10. New valve N-1 was measured for

comparison. A notable decrease in diameter, therefore material loss, in the used specimens is

most likely a result of wear. This can occur when combustion products and particulate matter

accumulate between the valve and the valve seating and cause abrasion.

Some of the used unbroken valves were bent upon arrival. It is unknown in the valves

were bent in service or during or after removal. The location of the bend in the valve is near the

head of the valve in all bent samples.

5.2 Non Destructive Test

Magnaflux dye-penetrant testing was performed on used valves to detect possible

surface cracks. No indications were observed in any of the used specimens. Used valves U-1

and U-2 are shown in Figures 10 and 11 respectively and are representative of results from all

12

used valves, presented in Appendix D. These results are consistent with ruling out the

possibility of low cycle fatigue because cracks have not formed at the surface. High cycle

fatigue, which is primarily seen in nonferrous alloys, is retained as a possible failure mechanism

due to the number of cycles that each exhaust valve incurs during its lifetime [6]. Estimating that

the valves are failing at 150,000 miles and the average number of cycles for each valve is 2000

cycles per minute (1 cycle per revolution at ~2000rpm) at a speed of 60 miles per hour yields a

lifetime of 3.0E8 cycles to failure [7].

More used specimens would have to be tested to look for surface cracks. The broken

samples need to be tested for cracks as well to determine if there are other cracks present on

the surface at the point of fracture or to determine if the fracture was localized.

5.3 Optical Microscopy

The head material was evaluated microscopically for broken, used and new specimens

and micrographs are shown in Figures 18-42 of Appendix F. The weld in all valves is located at

64 mm from the stem end. In all cases, the microstructure of the head was confirmed as an

austenitic phase steel with carbides. The microstructure of the stem material could not be

determined due to difficulty in resolving the fine grained martensitic lathe structure. In the used

and broken valves, the grain structure appeared to be finer at the edge of the valve than in the

center, which was confirmed by grain size analysis shown in Table 8.

At the interface, there is evidence of elongation of the grains in the austenitic steel. The

valves were likely friction stir welded although none of the examined valves failed at the weld. At

the weld interface there is evidence that there is diffusion from the austenite composition into the

ferrite composition and vice-versa, shown in Figure 43.

Grain size analysis was done according to ASTM E-112 using Abrams Three-Circle

Procedure. Results are shown in Table 8. The grain sizes of selected specimens were

determined at both the center of the valve stem and at the edge of each specimen. The grain

13

size at the edge of the valves is observed to be finer than that in the center for both new and

used specimens. This is likely due to the manufacturing of the valves.

The new valves N-1 and N-2 have relatively fine center grain sizes of 6.29 microns and

4.90 microns, respectively. As a general trend, the grains at the center of the used and broken

specimens are coarse relative to the new specimens, with a larger average diameter. This is

most likely a result of prolonged exposure to high temperatures during service up to 660 °C

causing grain growth.

The used valves investigated in this study were in service for an unknown length of time

before removal and therefore may have varying degrees of grain coarsening, which is an

important source of uncertainty in this analysis.

5.4 SEM/EDS

A compositional back-scatter electron (BSE) image at 100X magnification of the interface

between the head and stem materials in specimen B-2 is shown in Appendix E, Figure 12. The

image of the interface does not provide evidence of a compositional change in the materials

used in the VM exhaust valves. Figure 13 shows the head material at 1000X magnification. EDS

spectra of the precipitate phase, shown in Figure 14, identifies these as niobium carbides.

Semi-quantitative EDS results for the head material are shown in Table 5. The B-2 head

material composition most closely matches 21-4N valve steel, which is an austenitic stainless

steel widely used in diesel exhaust valve applications [8]. The composition of commercially

available 21-4N is shown in Table 6. Figure 15 shows the head material in a new valve.

Comparison between broken valve B-2 and new valve N-1 head materials show similar

elemental composition and carbide morphology. The SEM image and EDS spectrum of the

broken valve B-1 stem material are shown in Figures 16 and 17, respectively. Semi-quantitative

results for the stem material is shown in Table 7. This material is most likely a martensitic

stainless steel, as this is a commonly used material for valve stems. The failure site was

confirmed to be located at a position far from the weld in the austenitic phase. The stem material

14

in broken valve B-1 contains less chromium than typical martensitic stainless steels which

contain 12-15 wt.% chromium.

5.5 Carbon Content

Table 9: Carbon content of all sample conditions

Condition Sample Carbon Content

New 1 0.4163

New 2 0.4165

Used 1 0.4228

Used 2 0.4231

Broken 1 0.4146

The results of the carbon content are presented in Table 9. The carbon contents of all

sample conditions are very similar. Carbon content of the samples is not likely a significant

factor contributing to failure of the valves.

5.6 Vickers Hardness

Vickers microhardness was determined on the center and edges of the used, broken and

new specimens as shown in Table 10. The average hardness is 338.4 HV for used valves, 332.27

HV for broken valves and 349.62 HV for new valves. Used and broken valves have a hardness

that is overall less than new valves. This indicates that valves have softened while in service.

These results are in agreement with grain size measurements in Table 8, where the smaller grains

observed in new specimens correspond to higher hardness by the Hall-Petch relationship.

Hardness profiles were taken across the interface in broken specimens B-1 and B-2 (Figures 45

and 46). A decrease in hardness was observed at the interface between the softer austenitic head

and the harder martensitic stem.

15

5.6 Tensile Testing

Two used and one new specimen were tested in tension to generate stress-strain curves

and analyze fracture surfaces. One of the used tensile specimens was rendered invalid due to

equipment malfunction. The testing apparatus is shown in Figure 3, as previously noted in

section 4.8. The new valve, TN-1, exhibits a higher yield and tensile strength which were

measured to be 136 ksi and 168 ksi, respectively, as opposed to 112 ksi yield strength and 156

ksi tensile strength in the used sample TU-2.

The engineering and true stress-strain curves for TN-1 and TU-2 are plotted in Figures

47 and 48, respectively. The true stress-strain curves for both specimens are comparatively

shown in Figure 49. The used valve’s ductility increased to 4.9% total elongation at failure,

relative to the new valve, which failed at 2.5% total elongation.

The used valve has softened during its service life. Based on grain size measurements

in Table 8, used valves have, on average, larger grains than new valves. As stated above in

the hardness section, this results in softening from the Hall-Petch relationship.

Fracture surface analysis was performed on the test specimens shown in Figures 21

and 22. Both specimens failed in ductile overload with minimal necking. Radial marks

originating at the surface indicate that there was misalignment of the tensile load from the

central axis during testing.

6.0 CONCLUSION

The alloy characterized by our SEM/EDS analysis was discovered to be 21-4N stainless steel.

Although the size of the precipitates were quite large and could contribute to failure, this alloy is

a common diesel exhaust valve alloy and was therefore concluded to not be a factor in the

failure of these valves. The visible wear rings on the exhaust valves were not seen on the

intake valves that were removed from the same cylinder of the same engine. The wear rings act

16

as stress concentrators which could also contribute to failure. This is likely because failure was

found around the site were the wear rings were located. These wear rings may be a result of

engine design. The grain size of the in-service valves is much larger than the new valves, which

caused the material to decrease in hardness and strength throughout its service life. The

carbon content of the different valve conditions was not statistically different and did not have a

major contribution to failure.

Although ~3E8 cycles to failure is a relatively short life for this application, high cycle

fatigue was concluded to be a possible mechanism of failure. A future inspection of the exhaust

gas recirculation (EGR) equipment may provide further evidence for the cause of this shortened

valve life.

This investigation found no evidence that VM Motori has responded to reports of

exhaust valve failure by changing the design manufacturing process of the valves. The

characterization utilizing SEM/EDS elemental analysis and metallography of new and used

valves reveals no significant metallurgical differences.

Acknowledgements: We would like to thank Dr. Gerald Bourne for the opportunity and

support throughout the duration of this project and his love and passion for educating the next

generation of engineers. We would also like to thank Jim Hulse for providing specimens and

providing us with his experiential knowledge and background information about the problem.

Our work was supported by the Colorado School of Mines and the George S. Ansell

Department of Metallurgical and Materials Engineering.

17

APPENDIX A: INTRODUCTION/BACKGROUND FIGURES

Figure 1: Depiction of the underside of an engine head, which has been removed from the block,

where a valve head has sheared off and fallen into the cylinder

Figure 2: Image of the VM Motori R-428 DOHC diesel engine[2]

18

APPENDIX B: EXPERIMENTAL PROCEDURES FIGURES

Figure 3: Tensile sample TN-1 mounted in flat grips at the stem, on bottom, and a fabricated top mount that cradled the head of the valve

19

APPENDIX C: VISUAL INSPECTION FIGURES AND TABLES

Figure 4: Macrograph of fracture surface showing indications of high-cycle fatigue including ratchet marks around diameter and beach marks.

Figure 5: Macrograph of fracture surface showing severe damage from catastrophic failure.

20

Figure 6: U-1 after cleaning showing wear rings.



21

Table 4: Dimensional analysis along length of stem in used valves as compared to a new exhaust

valve shows evidence of material loss during the service life of the valve Distances from the surface of the valve head

Sample Avg. Diameter at Avg. Diameter at Avg. Diameter at Avg. Diameter at 0.75in (in) 1.00in (in) 1.25in (in) 0.75in (in)

U-1 0.232 0.232 0.222 0.221

U-2 0.231 0.232 0.223 0.222

U-3 0.232 0.230 0.220 0.221

U-4 0.233 0.233 0.222 0.222

N-1 0.232 0.233 0.227 0.227

Figure 9: Schematic illustration showing locations where diameters are measured.

22

APPENDEX D: NON-DESTRUCTIVE TESTING FIGURES

Figure 10: U-1 magnaflux results showing no strong indications of surface cracks.

Figure 11: U-2 magnaflux results showing no indications of surface cracks.

23

APPENDIX E: SEM/EDS FIGURES AND TABLE

Figure 12: Compositional BSE image of Figure 13: B-2 head material showing the presence of niobium carbides. interface between head material (above) and

stem material (below).

Figure 14: EDS spectra of carbides confirming composition as niobium carbides.

24

Table 5: Semi-quantitative EDS results for the bulk material composition of specimen B-2 head

material.

Table 6: Composition for 21-4N valve steel.

Element wt.%

Chromium, Cr 21.3 Manganese, Mn 9.0 Nickel, Ni 4.3 Niobium, Nb 0.38 Silicon, Si 0.25 max.

Figure 15: Compositional BSE of N-1 head material showing niobium carbides.

Figure 16: Compositional BSE image of B-1 stem material.

25

Figure 17: EDS spectrum of B-1 stem material.

Table 7: Semi-quantitative EDS results for the bulk material composition of specimen B-2 head material.

26

APPENDIX F: OPTICAL MICROSCOPY FIGURES AND TABLES

Figure 18: B-1 Center 20X Magnification.


27



28

Figure 22: B-2 Edge 20X Magnification

Figure 23: B-2 Edge 50X Magnification.

29



30



31



32

Figure 30: N-1 Center 20X Magnification.


33

Figure 32: N-1 Edge 20X Magnification.

Figure 33: N-1 Edge 50X Magnification.

34



35

Figure 36: U-1 Center 20X Magnification.


36



37

Figure 40: U-2 Edge 20X Magnification.

Figure 41: U-2 Edge 50X Magnification.

38



39

Table 8: Table of average grain size diameter from various locations in the austenitic phase, taken from the 20X magnification micrographs above.

Specimen Average Grain Diameter (μm)

B-1 Center 7.15 B-1 Edge 5.23 B-2 Center 11.55 B-2 Edge 6.42 B-3 Center 5.6 B-3 Edge 3.27 B-4 Center 8.2 N-1 Center 6.29 N-1 Edge 5.45 N-2 Center 4.9 N Weld 1 Center 6.52 N Weld 1 Edge 4.98 U-1 Center 6.23 U-2 Center 6.74 U-2 Edge 3.51 U-4 Center 8.17 UU1 Center 6.19 UU1 Edge 4.21 UU2 Center 6.97 UU2 Edge 4.12 U Weld Center 7.07 U Weld Edge 5.53

40

Figure 44: B-2 interface 20x Magnification with the austenitic valve head material at the top of the image and the martensitic stem material at the bottom of the image.

41

APPENDIX G: VICKERS HARDNESS TESTING FIGURES AND TABLES Table 10: Vickers hardness values for each specimen measured at the head with an overall average for

each specimen as well as the standard deviation.

Specimen

N Weld 1 Base by Weld

N Weld 1 Base

U Weld Stem

U Weld Stem by Weld

U Weld Base by Valve U Weld Base B-1

1 356.5660139 370.1426758 316.7965013 447.9746444 328.2704904 360.0161526 337.0484

2 325.293385 359.312996 315.0462444 439.1815081 339.1268727 353.7000797 328.2449

3 342.9196994 367.9365079 327.6788239 431.959003 344.233568 362.1493046 320.356

4 343.5982953 360.7150499 318.5816383 427.1000313 348.9387325 370.1485864 326.3812

5 334.2405508 353.7041276 350.2859863 420.7392887 336.4589079 358.6014851 341.7367

6 348.2689845 353.025388 325.2477418 430.680905 328.2286556 355.7888597 331.9823

7 327.6614542 368.667992 311.0292613 421.5369368 330.7129303 338.3324211 339.6262

8 345.0020837 353.7000797 322.1659335 431.605113 340.9299829 348.2375918 333.2183

9 348.9282274 348.9203489 328.2565442 424.2973376 347.5653184 354.3936093 327.0385

10 332.7591157 350.9568798 330.7129303 407.5156576 330.0830548 347.57053 337.0925 Avg. Hardness 340.523781 358.7082046 324.5801605 428.2590426 337.4548514 354.893862 332.2725

Std. Dev. 9.593736567 7.468607237 10.48275755 10.42984352 7.517900143 8.366763012 6.41089

42

Hard

ness

(HV)

450

400

350

300

250

Head Side Stem Side 200

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 Distance for interface (mm)

Figure 45: Vickers microhardness profile across interface of B-1, with the head side in the positive direction.

Hard

ness

(HV)

450

400

350

300

250

Head Side Stem Side 200

-10 -8 -6 -4 -2 0 2 4 6 8 Distance from Interface (mm)

Figure 46: Vickers microhardness profile from interface towards head in B-2, with the head side in

the positive direction.

43

APPENDIX H: TENSILE TESTING FIGURES

Stre

ss (k

si)

200 180 160 140 120 100 80

60

40

20

0

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 0.055 0.06 Strain (in/in)

Eng Stress-Strain True Stress-Strain

Figure 47: Engineering and true stress-strain curves for tensile specimen TN-1.

Stre

ss (k

si)

180 160 140 120 100 80

60

40

20

0

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Strain (in/in)

Eng Stress-Strain True Stress-Strain

Figure 48: Engineering and true stress-strain curves for tensile specimen TU-2.

44

True

Str

ess (

ksi)

180 160 140 120 100 80

60

40

20

0

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 0.055 0.06 True Strain (in/in)

TN-1 TU-2 Figure 49: True stress-strain curves comparing the tensile properties of specimens TN-1 and TU-2.

Figure 50: Fracture surface from the valve head side of the TN-1 valve.

45

Figure 51: Fracture surface from the valve head side of the TU-2 valve.

46

REFERENCES [1] "R 428 DOHC - 4 Cylinder Engine Euro 5 Version: Advanced Technology. - VM

Motori."English (UK). N.p., n.d. Web. 26 Apr. 2017. <http://www.vmmotori.com/r-

428-dohc/automotive-en/4-cilindri/r-428-dohcen.html>. [2] "Diesel Engine / 4-cylinder A 428 DOHC Vm Motori." DirectIndustry - The Online

Industrial Exhibition. Directindustry, n.d. Web. 26 Apr. 2017.

<http://www.directindustry.com/prod/vm-motori/product-60208-903629.html>. [3] "Metallographic Specimen Etching." Metallographic Etchants. N.p., n.d. Web. 26 Apr.

2017. <http://www.metallographic.com/Etchants/Etchants.htm>. [4] ASTM E92-16, Standard Test Methods for Vickers Hardness and Knoop Hardness of

Metallic Materials, ASTM International, West Conshohocken, PA, 2016, www.astm.org [5] ASTM E8 / E8M-16a, Standard Test Methods for Tension Testing of Metallic

Materials, ASTM International, West Conshohocken, PA, 2016, www.astm.org [6] http://www.vmmotori.com/r-428-dohc/automotive-en/4-cilindri/r-428-dohcen.html

[7] Lewis, Roger. "Wear of Diesel Engine Inlet Valves and Seats." Thesis.

University of Sheffield, 2000. Print. [8] Folkhard, Erich. Welding Metallurgy of Stainless Steels. New York: n.p., 1984. Print [9] Kearns, W. H. Welding Handbook. 7th ed. Vol. 3. Miami, FL: American Welding Society, [11] Dieter, George E. Mechanical Metallurgy. New York: MacGraw-Hill, 1986. Print. [12] Campbell, F. C. Elements of Metallurgy and Engineering Alloys. Materials Park: n.p.,

2008. Print. [13] Shirzadi, Amir. "Diffusion Bonding." Diffusion Bonding. N.p., n.d. Web. 26 Apr.

2017. <http://www.phase-trans.msm.cam.ac.uk/2005/Amir/bond.html>.

Documents

Metallurgical Investigation of VM Motori Exhaust Valves Vm...2.2 Applications The VM Motori model R-428 diesel engine was developed to be an affordable diesel engine for the general