Welding Behaviour of Low Nickel Chrome-Manganese Stainless

1361 © 2014 ISIJ

ISIJ International, Vol. 54 (2014), No. 6, pp. 1361–1367

Welding Behaviour of Low Nickel Chrome-Manganese Stainless Steel

Himanshu VASHISHTHA,1) Ravindra Vasantrao TAIWADE,1)* Rajesh Kisni KHATIRKAR,1)

Avinash Vijay INGLE1) and Ravin Kumar DAYAL2)

1) Department of Metallurgical and Materials Engineering, Visvesvaraya National Institute of Technology (VNIT), Nagpur, 440 010India. 2) Formerly Indira Gandhi Centre for Atomic Research. Now at Privately Working as Corrosion and MaterialsConsultant, Chennai-600100 India.

(Received on September 27, 2013; accepted on January 9, 2014)

Chrome-Manganese steel is relatively new steel as compared to its counterpart 304 series stainlesssteels. There are relatively few studies on the welding behaviour of low nickel Cr–Mn stainless steel (inparticular, on the effect of heat input on the microstructural developments). In the present investigation,a low nickel chrome-manganese stainless steel was welded (shielded metal arc welding process) to seethe effect of heat input on the microstructural evolution and mechanical properties. At higher heat inputs(404.2 J/mm and 528.1 J/mm), tensile strength and hardness are lower compared to low heat input(316.6 J/mm). Fractographic investigation of the tensile tested specimen revealed dimple-like ductile frac-ture. An attempt was also made to evaluate the phases incorporated in the investigated steel usingSchaeffler diagram.

KEY WORDS: Cr–Mn stainless steel; microstructural evolution; tensile strength; hardness; shielded metalarc welding; fractography; Schaeffler diagram.

1. Introduction

Nickel (Ni) prices have been relatively high worldwideover the last couple of years. This has led to a situation inwhich, the cost of Ni plays a significant role in the total costof stainless steel (SS) production. As a result, there has beenincreased interest in low-nickel or no-nickel grades ofstainless steel having properties similar to 18Cr–8Ni (18%Chromium and 8% Nickel) stainless steel.1,2) The AISI-200series SS is a well-known example of low-Ni stainless steelalloyed with manganese (Mn) and the other alloying ele-ments like nitrogen (N) and copper (Cu). In order to stabilizeaustenite phase, manganese acts as a substitute of nickel.3,4)

These low-nickel stainless steels are economical than 300-series and are popularly known as chrome-manganese stain-less steel (Cr–Mn SS).5,6) Its current contribution in totalstainless steel production is more than 10%.7,8) Low nickelCr–Mn SSs are used in various applications like homeaccessories, home appliances, light poles, construction, out-door installation etc. (where high corrosion resistance is notrequired).3,9,10)

Generally the Schaeffler diagrams are used as an impor-tant tool for predicting the constitution of stainless steels byevaluating Cr and Ni equivalents.11) These diagrams areused to represent the effect of proportion of elements on thestructure obtained after solution annealing.

According to latest information the 200-series SSs are thefastest growing steels and the future will place greater

demand on low-Ni alloys in replacement of 300-series SSswith respect to mechanical properties. The shielded metalarc welding (SMAW) is one of the most frequently used fab-rication technique for joining stainless steels like AISI 301,AISI 304 and AISI 316 SSs etc. As new economical engi-neering materials like Cr–Mn SS are developed freshly, it isimportant to cultivate the method of fabrication and fillermaterials for them.12,13) It is a well-defined fact that amongall welding parameters in SMAW process, welding current(I) is the most influential parameter because it affects the cur-rent density and hence the rate of filler and base material.14,15)

This welding current influences on mechanical and micro-structural properties by altering its weld pool and heat affect-ed zone (HAZ) width. There are numerous studies on theeffect of welding and its parameters on 300-series stainlesssteels.6,16) Subodh Kumar et al.16) worked on multipass weld-ing of 304 SS. They used gas tungsten arc welding (GTAW)process with three different heat inputs (2.563 kJ/mm, 2.78kJ/mm and 3.017 kJ/mm respectively) and studied micro-structural evolution and mechanical properties (like tensilestrength and hardness). Their results indicated that lowerheat input (2.563 kJ/mm) was superior to others for joining(but it should be sufficient for fusion of filler material andbase material). R. V. Taiwade et al.17) made a systematiccomparison of welding performance between AISI 304 andCr–Mn SS using electrochemical studies. The effect of sin-gle, double and triple pass of welding on HAZ, sensitizedzone (SZ-chromium carbides precipitated at grain boundar-ies) and tensile strength was investigated. The degree of sen-sitization (DOS) increased with increase in number of passes

* Corresponding author: E-mail: [email protected]: http://dx.doi.org/10.2355/isijinternational.54.1361

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ISIJ International, Vol. 54 (2014), No. 6

and highest DOS was obtained for triple pass welding ofCr–Mn SS. But a systematic characterization of low nickelCr–Mn SS before and after welding is until an untouchedarea. Therefore the present work is focused on the effect ofheat input (single pass welding by SMAW process) onmicrostructural evolution and mechanical properties of lowNi Cr–Mn SS.

2. Experimental Procedure

A low nickel Cr–Mn stainless steel was procured fromlocal market in the form of a rolled sheet. The chemicalcomposition of the steel is determined by optical emissionspectrometer which is given in Table 1.

Three identical samples of dimension 150 mm × 50 mm ×6 mm (length × width × thickness) were obtained from thegiven sheet with the help of wire-cut electrical dischargemachine (EDM). These samples were then solution annealedin silicon carbide (SiC) muffle furnace (Lenton, UK) at atemperature of 1 050°C for 90 minutes followed by waterquenching. Before submitting to welding, the oxide layer(which was formed due to rapid quenching) of samples wasremoved by polishing with the help of coarser emery papers(180 and 320 grit).

Bead-on-plate shielded metal arc welding (SMAW) wasperformed along the centre line of polished solutionannealed samples using “AWS E - 308 - 16” electrode hav-ing diameter 3.15 mm.18) Usually after welding there aredifferent zones formed in welded plates. The schematic rep-resentation of welded zones is given in Fig. 1.

BM- Base metalHAZ- Heat affected zonePMZ- Partially melted zoneFZ- Fusion zoneWZ- Welded zone

In order to study microstructural and mechanical proper-ties, three different heat inputs were selected. First plate waswelded at low heat input (85 A and 30 V), second at mediumheat input (100 A and 35 V) and third at high heat input(115 A and 40 V) by a skilled welder. These heat inputswere evaluated using Eq. (1) (considering arc efficiency, η =0.7)19) and are given in Table 2.

........................... (1)20)

Where, “V” is arc voltage in volts (V),“I” is welding current in amperes (A),“v” is welding speed in mm/s.

After bead-on-plate welding the samples change theirchemistry and formed different zones in SS plate. The weld-ed samples with low, medium and high heat input is shownin Fig. 2.

From these welded plates, samples for microstructuralexamination, tensile tests and micro hardness were carefullyobtained with the help of wire cut EDM. The schematic rep-resentation of obtained samples is shown in Fig. 3. In orderto avoid the starting and end effects of welding heat, 5 mmpart was discarded with the help of wire cut EDM from bothside of welded plates. In order to find out the strength ofwelded joint the tensile specimen of dimension 150 mm ×5 mm × 6 mm was taken out from the welded plates (low,medium and high heat input) in crossways direction. All flattensile specimens were prepared according to ASTM E 8Mstandards.21) Specimen was then fractured on tensile testingmachine “INSTRON 4467”. The maximum capacity of ten-sile testing machine was 30 KN.

For metallographic examination, a sample of facet 50mm × 10 mm × 6 mm was obtained from the centre of theweld in transverse route of weld direction from low, mediumand high heat input plates. The open surface (50 mm × 6mm) was polished on series of emery paper (180, 320, 600,

Fig. 1. Schematic showing the formed zones after welding.

Table 2. Welding parameters and heat input values.

Specimen Current (A) Voltage (V) Welding speed(mm/s)

Heat input per unitlength (J/mm)

Sample 1 85 30 5.637 316.6

Sample 2 100 35 6.060 404.2

Sample 3 115 40 6.097 528.1

Q V I v= ( )η /

Table 1 Chemical composition (wt.%) of low nickel Cr–Mn stainless steel.

Element C Si Mn P S Cr Mo Ni Al Cu Co

(Wt.%) 0.073 0.531 4.412 0.086 0.005 14.060 0.252 2.701 0.004 1.233 0.114

Fig. 2. Actual welded sample a) low heat input; b) medium heat input and c) high heat input.


1363 © 2014 ISIJ

800 grit) and finally cloth polished with alumina slurry (0.75μm). Subsequently the sample was ultrasonicated in dis-tilled water prior to etching. A Potentiostat (Solartron-1285)was used galvanostatically as per ASTM standard A-262Practice A test. In Practice A test, the sample was electro-lytically etched in 10 wt.% oxalic acid solution with 1 A/cm2

current density for 90 seconds over the surface area of 10mm × 6 mm and the remaining area was insulated using tef-lon tape.22)

3. Results and Discussion

3.1. Phase IdentificationIn order to find out the phases present in low nickel Cr–

Mn SS before submitting to welding, Schaeffler diagram23)

was used in this investigation as shown in Fig. 4. Nickelequivalent (Niequi) and chromium equivalent (Crequi) werecalculated using given equations.23)

.......................................... (2)

.......................................... (3)

In Schaeffler diagram the graph is plotted between Niequivalent and Cr equivalent (Crequi as abscissa and Niequi asordinate). Calculated Niequi (7.58) and Crequi (15.52) werethen located in Schaeffler diagram. The point in blue is posi-tioned inside the red rectangle of graph in Fig. 4, whichshows the actual phase composition of the investigated steel.According to this diagram it is evident that the studied lownickel Cr–Mn stainless steel contains an austenitic phase ina very small amount (as blue point situated at the transitionline of A+M+F region and M+F region). Phases identifiedby Schaeffler diagram were then cross checked by opticalmicrograph of solution annealed sample (10 mm × 10 mm ×6 mm) which is shown in Fig. 5. The open surface (100 mm2)of sample was polished on series of emery paper (180, 320,600, 800 grit) and finally cloth polished with alumina slurry(0.75 μm). Subsequently the sample was ultrasonicated indistilled water prior to etching. Then the sample was marbleetched for 10 seconds and observed under optical micro-scope (Zeiss Axiolab). It is evident from obtained micro-graph that the steel under investigation is consisting ofphases like austenite; ferrite and martensite are in good

agreement with Schaeffler diagram.

3.2. Effect of Heat Input on HAZIn welding operation, current (I) is one of the most influ-

ential parameter, because it affects the current density andhence the rate of filler and base material. The welding cur-rent is responsible to the formation of various zones includ-ing weld zone, fusion zone, partially melted zone, area nearto the partially melted zone (i.e. HAZ) and unaffected basemetal (see Fig. 1). The effect of heat input on HAZ wasevaluated in terms of width and measured using image anal-yser (Zeiss Axiolab) was obtained as 118.79 μm, 131.52 μmand 138.08 μm for low, medium and high heat inputsrespectively. It was observed that as the heat input (especial-ly welding current, I) increases the width of HAZ alsoincreases which is shown in Fig. 6.

When heat input value increased, the cooling ratedecreased significantly and a sufficient time is available forgrain growth. This grain coarsening leads to a great incre-ment in sensitized zone (SZ) and HAZ and hence the trueHAZ (sensitized zone + heat affected zone) increased sub-sequently for higher heat inputs and quantitatively given inTable 3.

The increase in width may be attributed to the formationof chromium carbides at the grain boundaries. Sensitized

Fig. 3. Schematic representation of samples obtained for metallo-graphic examination and mechanical properties.

Cr equi Cr Si Mo V Al

Nb Ti

= + + + +

+ + +

( ) ( ) ( ) ( ) ( )( ) ( )

2 1 5 5 5 5

1 75 1 5 0

. .

. . ..75 W( )

Ni equi Ni Co Mn Cu N C = + + + + +( ) ( ) ( ) ( ) ( ) ( )0 5 0 3 25 30. .

Fig. 4. Schaeffler diagram used for investigated low nickel Cr–Mnstainless steel.

Fig. 5. Optical micrograph of solution annealed low nickel Cr–Mnstainless steel obtained at 200×.

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zone was further verified by using “SEM JEOL 6380” asshown in Fig. 7 (image a, b and c). Few traces of chromiumcarbides were observed for low heat input and the densityincrease for higher heat inputs subsequently. R. V. Taiwadeet al.17) also worked on welding behaviour of low nickel Cr–Mn austenitic stainless steel and found that the width of trueHAZ is about 5 mm for single pass SMAW with 766 J/mm

Table 3. Measurement of HAZ, SZ and true HAZ for different heatinputs.

Heat input HAZ (μm) SZ (μm) True HAZ (μm)

Low 62.10 56.69 118.79

Medium 67.45 64.07 131.52

High 70.91 67.17 138.08

Fig. 6. Effect of heat inputs on true HAZ a) low heat input; b) medium heat input and c) high heat input.

Fig. 7. SEM micrographs of sensitized zone of true heat affected zone a) low heat input; b) medium heat input and c) highheat input.


1365 © 2014 ISIJ

heat input (75A, 35V and 2.57 mm/s). The width of trueHAZ of studied steel is very small and found to be 138.08μm for high heat input because the austenitic phase (evidentfrom Figs. 4 and 5) presents in a small quantity and there-fore only few sites of austenite is available for chromiumcarbide precipitation.

3.3. Effect of Heat Input on Welded ZoneTypically the dendrites decided the mechanical properties

of welded joint. In the area of weld zone, only dendritesstructure is present. We know that as the heat input valueincreases, the cooling rate decreases respectively.16,19) It hasbeen observed that higher the cooling rate, shorter the solid-ification time and finer the dendrites structure. Dendritesarm spacing is the function of cooling rate or solidificationtime can be expressed by Eq. (4).24)

........................... (4)

Where,

‘d’ is the dendrites arm spacing,‘tf’ is the local solidification time,‘ε’ is the cooling rate and,‘a’ and ‘b’ are proportional constant.In the above equation dendrites arm spacing is directly

proportional to solidification time, and inversely proportion-al to cooling rate. This dendrites growth phenomena duringsolidification is given microstructurally in Fig. 8.25)

The effect of low, medium and high heat input on den-drites structure of investigated steel is shown in Figs. 9(a),9(b) and 9(c) respectively.

Micrographs in these images show that, as the heat inputincreases, sufficient dendrites growth was observed becauseof slow cooling rate. During solidification large dendritesarms grow at the expense of smaller ones. The slower thecooling rate during solidification, the longer is the time avail-able for coarsening and larger the dendrites size and dendritesarm spacing. For confirmation of dendrites growth, weldzone was further characterized using “SEM JEOL 6380”and shown in Figs. 10(a), 10(b) and 10(c) respectively.

From SEM results it was also found that, the dendritespacing is less at the top surface of the weld pool andincreases towards the base metal. This is because the cool-ing rate is higher at the top surface (welding line) anddecreases towards the base metal. The dendrite length anddendrite spacing was measured using image analyser (ZeissAxiolab) is given in Table 4. The relationship between dif-ferent heat inputs and dendrites size and spacing is present-ed in Fig. 11. The graph of the figure clearly indicates thatthe dendrite size and their spacing increases continuously

Fig. 8. Schematic representation of dendrites growth.

d at bfn n= = ( )−ε

Table 4. Dendrite measurement for different heat inputs.

Properties Low heat input Medium heat input High heat input

Dendrite length (μm) 126 198 325

Dendrite spacing (μm) 23 28 38

Fig. 9. Effect of heat input on dendritic structure of welded zone a) low heat input; b) medium heat input and c) high heatinput.

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with increasing heat input. Also it is known that alterationin dendritic size and spacing directly affects the mechanicalproperties of welded joint. This is the motivation to studythe tensile strength and the hardness of welded joint in thisinvestigation.

3.4. Tensile Strength and FractographyTensile strength for low, medium and high heat input

samples was evaluated with the help of tensile testingmachine. Samples were loaded one by one at tensile testmachine and force was applied, elongation takes placeinside the sample and after some time it failed with a noisysound. The maximum tensile strength was obtained for lowheat input (650.81 MPa) and further it was prominentlydecreases with increase in heat input values (622.54 MPaand 569.32 MPa for medium and high heat input respective-ly). Small dendrite size and spacing attributed to high tensilestrength and ductility for low heat input sample than others.Fractured samples were further analysed using SEM andfractograpghy results shows that all samples were failedwithin true HAZ region. Figures 12(a), 12(b) and 12(c)shows SEM fractographs of low, medium and high heatinput respectively. Most of the failure took place in ductilemanner and evident from dimple-like structures of fracturedtensile samples.

3.5. MicrohardnessMicrohardness measurement was carried out in transverse

direction (i.e. perpendicular to weld centre line). Figure 13Fig. 11. Effect of different heat inputs on dendrites length and

spacing.

Fig. 10. SEM images of welded zone a) low heat input; b) medium heat input and c) high heat input.

Fig. 12. SEM images of fractured cross section for different heat inputs.


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shows the hardness curve for low, medium and high heatinput for the indenter movement from weld zone towardbase metal via. fusion zone, partially melted zone and heataffected zone. The microhardness graph is plotted on bilat-eral symmetry because most of the times hardness value issame on both side of the centre line at same distances. Hard-ness varies from 183.9 to 223.5 VHN for high heat input,195 to 228.3 VHN for medium heat input and 208 to 243.4VHN for low heat input. Highest hardness values for low,medium and high heat inputs were found 261, 255 and 251VHN which can be attributed to the partially unmeltedgrains in PMZ (which are partially adopted as nuclei duringsolidification of welded joint). Afterwards samples showsdecreasing trend of hardness in HAZ region due to graincoarsening. In HAZ, region near to PMZ subjected to slowercooling rate and resulted in coarse grains whereas areatowards the base metal having faster cooling resulted in finegrain structure.

4. Conclusion

(1) Optical micrograph of solution annealed sample wascross checked by schaeffler diagram and it was concludedthat steel consist of phases like austenite (in small amount),ferrite and martensite.

(2) Width of heat affected zone increases with increasein heat inputs. Accordingly considerable alterations in grainsize (grain coarsening) were found and cause HAZ growth.

(3) Maximum tensile strength possessed by low heatinput welded joint due to smaller dendrites and low spacing.

(4) Most of the fracture takes place in a ductile mannerfor low, medium and high heat inputs.

(5) Microhardness varies from centre of the weld to thebase metal in an increasing manner, some different trendswere also found in hardness profile because of PMZ forma-tion (hardness is very high at partially un-melted region).

(6) Failure takes place inside the true HAZ, this meansthat tensile strength of weld joint is good enough and forwelding of 6 mm thick plate, SMAW offers a wide range ofparameters to the fabricator for low nickel Cr–Mn SS.

AcknowledgmentThe authors would like to thank Director Dr. N. S.

Chaudhari, VNIT Nagpur for providing the necessary facil-ities for carrying out this investigation and for his constantencouragement to publish this work. The authors are alsograteful to Miss Adeeba Khan (Research Scholar, CorrosionEngineering Laboratory, Department of Metallurgical andMaterials Engineering) for her help in conducting electro-chemical tests, Mr. Shreedhar Gadge (Senior Technician,Chemical Analysis Laboratory Department of Metallurgicaland Materials Engineering) for performing solution anneal-ing treatment. The authors would also like to thanks Mrs.Varsha Patankar (Technical staff, Testing of MaterialsLaboratory, Department of Metallurgical and MaterialsEngineering) for her help in conducting mechanical testing.

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Fig. 13. Microhardness profile showing hardness variation incrossways direction of weld joint.

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Welding Behaviour of Low Nickel Chrome-Manganese Stainless