Hot Ductility and Hot Cracking Behavior of Modified 31 6 Stainless Steels

Designed for High-Temperature Service

Test results on modified 316 stainless steels correlated well with those on conventional 316 materials and fully austenitic 316 stainless steels

BY C. D. L U N D I N , C. Y. P. Q I A O , T. P. S. GILL A N D G. M . G O O D W I N

ABSTRACT. The weldability of the modi­fied 316 stainless steel was evaluated by the Cleeble hot ductility test and two hot cracking test methods (Varestraint and Sigmajig). The fusion zone and weld metal heat-affected zone (HAZ) hot cracking susceptibilities of the modified 31 6 stainless steel are similar to conven­tional fully austenitic 316 stainless steels and greater than the conventional 31 6 materials that have a primary ferritic so­lidification mode. The Cleeble hot duc­til ity test results correlate with the base metal HAZ hot cracking results from the Varestraint test and indicate that the modified 316 materials show a consid­erably higher base metal HAZ hot crack­ing susceptibility as contrasted to nu­clear grade 316 stainless steels. Vare­straint test results and Sigmajig test re­sults for the tested materials showed good correlations.

The sensitivity of the base metal to HAZ l iquation cracking has been suc­cessfully predicted by using a newly de­veloped hot ductility criterion, the ratio of ductility recovery (RDR). An excellent correlation between the Gleeble Test cri­terion RDR and the Varestraint Test cri­teria (TCL, MCL and CHL) has been found.

Introduction

The development of an advanced austenitic alloy to meet the needs of new generating units (650°C (1200°F)/35

C. D. LUNDIN, C. Y. P. QIAO, and T. P. S. GILL are with Materials Science and Engi­neering Department, University of Ten­nessee, Knoxville. C. M. GOODWIN is with Metals and Ceramics Div., Oak Ridge Na­tional Laboratory, Oak Ridge, Tenn.

MPa (5.1 ksi)) is being aggressively pur­sued in Japan and Europe, and in the U.S., the Department of Energy, through the AR&TD Fossil Energy Materials Pro­gram managed by Oak Ridge National Laboratory (ORNL), has extensive stud­ies underway. The basic tenets of alloy design involve long-time strength and creep rupture properties, metallurgical stability, corrosion resistance and fabri­cability (including weldability).

Modif ied 316 stainless steels (lean 316 stainless steels) have been recently developed at ORNL based upon the compositions of materials used for nu­clear applications. The basic modifica­tion is achieved by using carbide-form­ing elements and solid solution elements plus a thermomechanical treatment con­sistent with superheater tube production procedures to improve the elevated-tem­perature properties. ORNL proposed cri­teria for design of new alloys (Ref. 1) and pointed out that the added strength is achieved through the precipitation of fine carbides, nitrides or phosphides that

KEY WORDS

Hot Ductility Hot Cracking Modified 316 SS High Temp. Service Austenitic SS Gleeble Testing Varestraint Testing Sigmajig Testing Testing Criteria HAZ Cracking

stabilize dislocation networks. Extensive research work has been accomplished on the mechanical properties of the modified 316 materials (Refs. 2-4). The work at ORNL and Cornell University showed that the modified 316 materials possess superior f low strength at high temperature compared to conventional 316 stainless steels. The high strength and ducti l i ty is produced by reducing Cr, increasing Ni and C, adding combi­nations of carbide-forming elements that include Ti, Nb and V; adding ES, permit­ting P to reach 0.04%; and introducing enough cold work to assure a minimum yield strength of 300 MPa (44 ksi). Todd (Ref. 5) evaluated the microstructure of the modified 316 materials and con­cluded that the improved creep rupture strength is due to the presence of car­bides and phosphides and the precipi­tate distribution. Two types of MC car­bides were observed: a titanium-rich MC characteristic of the original anneal and a Ti-V-Mo-rich MC that precipitates on both matrix dislocations and at grain boundaries.

Fabrication procedures, especially by welding, must fall w i th in the realm of standard practice in the fossil energy in­dustry and be consistent with the tenets of the ASME Boiler and Pressure Vessel Code. The alloys should be free from hot cracking, reheat cracking, and HAZ creep rupture property deterioration; ex­hibit freedom from embritt lement in long-time service; be capable of being joined to other alloys without long-time interface effects; and the filler metals for joining the alloys should be developed concurrently with the alloy selection in­vestigations.

This paper presents the results from hot ductility and hot cracking tests con-

W E L D I N G RESEARCH SUPPLEMENT I 189-s

Table 1 -

C Si Mn \ i

Cr Ti Nb V Mo P Ii S N Cu Al Co

- Composition for Current Study

Modified 316 (AX6/CE3890)

0.079 0.27 1.77

16.87 14.29 0.21 0.10 0.52 2.27 0.040 0.006 0.010 0.012 0.02 0.006

<0.01

Materials (wt. %)

Modified 316 (L316BW)

0.085 0.13 1.86

15.59 14.22 0.12 0.19 0.51 2.21 0.028 0.005 0.004 0.0082

<0.05 0.036

—

Reference 316

0.057 0.58 1.86

13.48 17.25 0.02

<0.01

— 2.34 0.024 0.005 0.019 0.03 0.1 0.023 0.02

316 NC-1

0.010 0.46 1.09

11.50 17.40

— — —

2.88 0.021

— 0.001 0.105

— — —

316 NG-2

0.010 0.51 1.60

12.95 17.55

— — —

2.76 0.021

— 0.001 0.113

— — —

347-1

0.045 0.54 1.60 9.71

17.80

— 0.67

— —

0.032

— 0.007

— — — —

ducted on modified 31 6 steels and a Ref­erence 31 6 during weldabi l i ty studies conducted at The University of Ten­nessee. Since hot cracking sensitivity is a major issue in the weldability of stain­less steels, an understanding of the hot cracking sensitivity of the newly devel­oped stainless steels is a requirement for acceptance by the utility industry.

Generally, hot cracking can be clas­sified into base metal liquation cracking and weld metal solidification cracking. The Gleeble hot ducti l i ty test and the Varestraint hot cracking test are widely used testing methods for the evaluation of hot cracking susceptibility. The duc­t i l i ty behavior of a material obtained from the Gleeble test reflects the mate­

rials capability of enduring the welding thermal strains in the HAZ at elevated temperature. The Varestraint test is con­sidered an efficient laboratory-type test­ing method by which hot cracking in­formation on the fusion zone, weld metal HAZ and base metal HAZ can be obtained. The newly developed Sigma­jig hot cracking test also showed excel­lent capabilities as a candidate for a fu­sion zone hot cracking evaluation method. All of these testing methods have been successfully employed to evaluate the weldability of nuclear grade stainless steels and have been used in this research in like manner on the modi­fied 316 materials.

Table 2 — Hot Ductility Testing Parameters

Parameter

Sample

Thermocouple

Thermal Cycle

On Cooling Peak Temperature

I lold Time

Crosshead Speed law Separation

Testing

Conditions

Diameter: 6.35 + 0.025 mm (0.25±0.001 in.) Cylindrical; Length: 102 ± 3 mm (4.0 + 0.125 in.) Thread: '/i-20 on both ends Diameter: 0.254 mm (0.010 in.) Chromel-Alumel: <1371°C (2500°F) Nobel metal/alloy: >1371°C (2500°F) Attachment methods: percussion/spot welding

separate wire technique Characteristic of a SMAW weld in 38-mm (1'/2-in.)

thick stainless steel with an energy input of 2.8 kj/mm (70 kj/in.) at 22?C (72 = F) preheat

Zero ductility or zero strength temperature Test temperature1'"': 1-2 seconds permitted Peak temperature: Not permitted 63.5+13 mm/s (2.5±0.5 in./s) 20 ±5 mm (0.8 ±0.2 in.) Minimum of two tests at each temperature. A

difference of greater than 30".. in reduction in area, will necessitate one more test.

Testing temperature intervals: 56-111 °C (100-200 °F) intervals below ZDT-56°C (ZDT-100°F) or ZST-56 = C (ZST-100°F); 14 C° (25 F°) intervals between ZDT and ZDT-56°C (ZDT-100=F) or ZST and ZST-56 C (ZST-100=F)

(a) Only when lest temperature is below ZDT-56 C (ZDT-100 F). ZST-56 C (ZST-100 F)

Materials and Experimental Procedures

The chemical composition of the 316 materials, including two heats of the modified 31 6 and one heat of Reference 31 6 stainless steel, is listed in Table 1. The chemical composition of two heats of nuclear-grade 316 and one heat of AISI 347 stainless steel is also included in Table 1 for comparison. The two heats of the modified 316 (AX6/CE 3890 and L31 6BW) contain higher amounts of C, Ti , Nb, V and P compared to conven­tional standard 31 6 stainless steel. The ferrite potential for all three heats is less than zero and hence they exhibit a fully austenitic microstructure. These alloys were produced in tubular form by Com­bustion Engineering and Babcock and Wi lcox (2.5-in. O.D.X 0.27-in. wall/63.5 X 6.86 mm). The Combustion Engineering alloys were centrifugally cast as 5.125-in. O.D.X 1.1 25-in. wal l (130 X 29 mm) and then reduced. The Babcock and Wilcox lean 316 stainless steel was extruded using conventional commercial practices.

The hot ductility response of the ma­terials was evaluated by Gleeble testing employing the newly standardized Glee­ble hot ductil ity testing parameters (the outcome of a detailed study carried out at The University of Tennessee (U.T.) (Ref. 6). The standard test conditions are given in Table 2.

The alloys in this program were also evaluated for their resistance to hot cracking using two different test meth­ods, namely, the Varestraint test and Sig­majig test. The Varestraint test is a widely used laboratory-type test method and permits the evaluation of fusion zone weldability as well as that of the fusion zone/base metal/heat-affected zone. During the test, a weld bead is deposited to simulate the thermal conditions rep-

190-s I M A Y 1993

tD Q3SSBB S3? I a -Ji1

Top View W a d g a - * *

Spec iman al ter Band •

Specimen before Band •

GTAW To rch At tached to DIG B lock Suppor t B lock to Move w i t h It

Front View

A

MHa

MBBBIM

li|iii|!|::i|!iiiiii|!FiF i ' • i * i • i 2 i t 2 ° 24 28 i ± Si

4 ,

V s" **»

V \ +

\^u

_JIHMI

%mm*

9

w ? y

!

i IjJIIIH 4 4 1 2 , ^ 1

-5 .-. ••: -. UJ :

Fig. 1 — Schematic drawing of the Varestraint test device showing the manner in which a specimen is tested.

Fig. 2 — Photograph of tested Sigmajig specimens. A — Partially cracked sample; and B — completely cracked sample.

resentative of the heat input of a fabri­cation weld. Therefore, the microstruc­tures in the sample are virtually identi­cal to those encountered in actual pro­duction welds. Mechanical restraint is simulated by an externally applied aug­mented strain. The dimensions of Vare­straint samples used in the study are 5 X 1 X 0.125 in. (127 X 25.4 X 3.2 mm).

Figure 1 shows a schematic drawing of the Varestraint test device and a macroscopic sketch of the tested region. The U.T.-modified multipass technique, in which the fourth and fifth additional beads are superimposed directly on the third pass, was employed during the test­ing (Ref. 7). The augmented strain is ap­plied during the fifth pass just after the torch passes the center of the radius block, such that the augmented strain is applied to the solidifying interface at the center of the sample directly over the crown of the radius block.

The welding conditions employed for Varestraint testing are indicated as fol­lows:

Process Autogenous GTAW Current 100 A Voltage 12V Travel speed 10 in./min. (4 mm/sec) Shielding gas Argon 30 ft3/h (0.85m3/h) (10 s pre-and post weld purge) Electrode 2% thoriated tungsten (%2-in. (2.4-mm) diameter 60-deg tip

angle) Arc length 0.062 in. (1.6 mm) Polarity DCEN Interpass temperature 72°F (22°C)

The Sigmajig test is a newly devel­oped laboratory-type test method (Ref. 8). During the test, a fixture holds a square specimen between hardened steel grips and a transverse stress is ap­plied prior to welding. After preloading, a GTA weld is produced along the spec­imen centerline. The Sigmajig specimen is a 50 X 50-mm (2 X 2-in.) square with a thickness of 1 mm (0.04 in.). The GTA welding conditions were 30 A with 0.89-mm (0.035-in.) arc length and a travel

speed of 15 mm/sec (35 in./min). Figure 2 shows macrographs of tested Sigma­jig specimens that were partially or com­pletely cracked during the test.

Results and Discussion

Hot Ductility Tests

Normally, the hot ductil ity behavior of a material exhibits a good correlation wi th HAZ hot cracking sensitivity and over the years many criteria have been developed to correlate these two behav­iors. Yeniscavich (Ref. 9) analyzed the different criteria, which can be summa­rized as follows:

1) Arbitrary minimum ductility 2) Recovery rate of ductility 3) Recovery rate of ultimate strength 4) Zero ductility range 5) Ductility dip temperature range None of the proposed criteria has

found universal acceptance because of variations in the critical testing parame­ters and the effect of microstructural fac-

W E L D I N G RESEARCH SUPPLEMENT I 191-s

< LU

CC

< -z. o h-

o ZJ D LU CE

5%

HOT DUCTILJTY ON-HEATING

ON-COOLING

DRR (%) = X100 AC

AREA(BCF) RDR (%) = : — X100

AREA(ACF)

NDR (F°) = DF

TEMPERATURE

Fig. 3 — Schematic illustration of the concept of Gleeble hot ductil­ity test criterion.

100

K

<

_ 8 0 "

60

40 "

20

HOT DUCTILITY OF REFERENCE 316

— ON HEATING - - ON COOLING FROM ZDT OF 2475 °F

1 7 0 0 1 9 0 0 2 1 0 0 2 3 0 0

(927-C) (1038°) (1149°) (1260°)

TEMPERATURE (°F)

Fig. 4 — Hot ductility behavior of Reference 316.

2 5 0 0

(1371°)

tors on hot ductility behavior. However, among the above, the recovery rate of ductil ity and zero ducti l i ty range crite­ria are widely used.

Upon cooling from a peak tempera­ture near the melting point, the ductility remains zero over a finite temperature range before it begins to recover. The temperature range from the melting point to the temperature where ductility

just begins to recover is termed the zero ductility range (ZDR). Since it is difficult to measure an exact ducti l i ty recovery temperature, another criterion, nil duc­tility range (NDR), proportional to ZDR, is usually employed and was employed in this study. The temperature for 5% ductility recovery is measured from the on-cooling hot ductility curve and is de­fined as the lower limit of the nil ductil­

ity temperature range. The NDR can be measured with better accuracy than the ZDR.

The Ducti l i ty Recovery Rate (DRR) criterion represents the extent of ducti l­ity recovery upon cool ing from the "peak" temperature. If the ducti l i ty re­covery is immediate, the material is con­sidered crack resistant and vice versa. The DRR is determined from the on-

100

o = > Q

HOT DUCTILITY OF MODIFIED 316 (AX6/CE 3890)

a — ON HEATING - - • - - ON COOLING FROM ZDT OF 2375°F

Fig.

2 3 0 0 2 5 0 0

(1260°C) (1371°C)

TEMPERATURE (°F)

Hot ductility behavior of modified 316 (AX6/CE 3890).

100

5 60 <

a LU

E

HOT DUCTILITY OF MODIFIED 316 (L316BW)

B— ON HEATING - - • - " ON COOLING FROM 2400'F

°~~' <* >v **' ** \

*» * \ • - « • \ \ .

* ' x* \ * I * \ \ \ % t

\ * \ \ I \ \ > \

L T *

1 6 0 0 1 8 0 0 2 0 0 0 2 2 0 0 2 4 0 0 (871°C) (982°C) (1093°C) (1204°C) (1316°C)

TEMPERATURE (°F)

Fig. 6 — Hot ductility behavior of modified 316 (L316BW).

192-s I M A Y 1993

heating and on-cool ing hot ducti l i ty curves as the ratio of percentage reduc­tion in area at an arbitrary temperature (DRR-1) or at a temperature where the on-heating ducti l i ty decreases rapidly (DRR-2).

The above hot ductility criteria, NDR, DRR-1 and DRR-2, were not found to provide acceptable correlations with the base metal hot cracking sensitivity of the materials in this study, therefore, a new criterion, Ratio of Ducti l i ty Recovery (RDR), was developed and evaluated. The RDR is calculated by determining the ratio of the areas under the on-cool­ing and on-heating curves from the zero ductil ity temperature to the on-heating ducti l i ty decrease temperature. The physical significance of the new con­cept is related to ducti l i ty recovery in the temperature range from the ZDT to temperature which is generally associ­ated with the formation of grain bound­ary liquation in the base metal HAZ (the ductility decrease temperature on heat­ing). On the contrary, the DRR can only reflect the ductility recovery for the spe­cific temperature at which it is deter­mined. Therefore, RDR appears to be more suitable as contrasted to the con­ventional criteria NDR and DRR in ex­plaining base metal HAZ hot cracking sensitivity. Obviously, if the RDR or DRR is small or the NDR is large, the mate­rial w i l l be more sensitive to HAZ hot cracking. The concepts of DRR, RDR and NDR are schematically illustrated in Figure 3.

The hot ducti l i ty response of refer-

Table 3 — Results of Gleeble Hot

Materials

Modified 316 (AX6/CE 3890) Modified 316 (L316 BW) Reference 316 316NC-1 316NG-2 347-1

Ductility Tests

DRR-1<a> (",',)

13 6

97 84 62

4

DRR-2<b> (%)

41 46 66

100 62

9

NDRW f

129 84 27 70 80

190

RDRWI (",,)

28 18 75 86 33 6

(a) Ductility recovery rate measured at 2300 F (b) Ductility recovery rate measured at ductility decrease temperature (c) Nil ductility temperature range (d) Rate of ductility recovery from zero ductility temperature to ductility decrease temperature

ence 316, modified 316 (AX6/CE 3890) and modified 316 (L316BW) are pre­sented in Figures 4, 5 and 6, respec­tively. In these figures, percent reduc­tion in area is plotted as a function of test temperature. The solid line repre­sents the on-heating behavior while the broken line represents the on-cool ing test results. The data spread for each test is described by a bar. The parameters, RDR, DRR-1, DRR-2 and NDR, deter­mined for each alloy from hot ductil ity response curves, are given in Table 3. It is evident that the hot ductility recovery of the modif ied 31 6 heats is apprecia­bly lower than that of the reference 316 heat. In order to compare the hot duc­t i l i ty behavior of modified 316 alloys with the "conventional" stainless steels, the RDR, DRR and NDR parameters for typical nuclear grade 31 6NG-1 (primary ferritic solidification mode) and 316NG-2 (primary austenitic solidif ication mode) are also provided in Table 3.

Since the modified 31 6 alloys contain microalloying elements, the hot ducti l­ity parameters in Table 3 are also com­pared with those of AISI 347 stainless steel.

Figures 7 and 8 show the hot ducti l­ity behavior of 31 6NG-2 and AISI 347 materials, respectively. It is evident that the modified 316 (AX6/CE 3890) and AISI 347 have almost identical on-heat­ing behavior. The hot ductility gradually decreases over a wide temperature range (200-300F795-1 50°C) as the tempera­ture approaches the ZDT. The modified 316 (L316BW) and 316NG-2 have very similar hot ducti l i ty behavior. The hot ducti l i ty curve for the Reference 316 heat shows the highest RDR and DRR ratios and lowest NDR among the ma­terials studied.

The microstructure typical of a hot ductility sample from the modified 31 6 (AX6/CE 3890) tested at the ZDT of 2400°F/1 31 5°C is shown in Fig. 9. It is

o i-o Q

80 -

6 0 -

4 0 -

2 0 -

n-

HOT DUCTILITY OF 316NG-2 (D4C1204)

° — ON HEATING

- - • - - ON COOLING FROM ZDT OF 2500°F

•—-""" *V

\ 1 \ \ 1

\ I i T \ I i 1 i 1 i I 1 o

i 1 t I t 1 i 1 i 1 i 1

n \ X 1

V 1

1 1 fc->

o

1700 1900 2100 2300 2500 (927°C) (1038"C) (1149°C) (126CTC) (1371X)

TEMPERATURE (°F)

60

40

20

HOT DUCTILITY OF 347-1 (876195)

i — ON HEATING

l - - ON COOLING FROM ZDT OF 2450°F

1 7 0 0 1 9 0 0 2 1 0 0 ( 9 2 7 ° C ) ( 1 0 3 8 ° C ) ( 1 1 4 9 ° C )

TEMPERATURE (°F)

Fig. 8 — Hot ductility beha vior of 317-1.

Fig. 7 — Hot ductility behavior of 316NG-2.

W E L D I N G RESEARCH SUPPLEMENT I 193-s

Fig. 9 — Microstructure of hot ductility sample of modified 316 (AX6/CE 3890) tested at ZDT of 2400°F.

• I n c i p i e n t l y M e l t e d G r a i n B o u n d a r i e s

Fig. 10 — Microstructure of hot ductility sample of modified 316 (L316BW) tested at on-cooling at 230CPC.

ev iden t that rup tu re o c c u r r e d in an i n ­te rgranu la r fash ion and the l o w d u c t i l ­i ty is m a i n l y d u e to g ra in b o u n d a r y l i ­q u a t i o n . Th is ind ica tes tha t H A Z ho t c r a c k i n g occu rs due to the decrease in grain boundary strength at elevated t e m ­perature coup led w i t h the H A Z thermal strains resul t ing f rom w e l d i n g . Figure 10 shows the m ic ros t ruc tu re o f a hot d u c ­t i l i t y sample of mod i f i ed 316 (L31 6BW) tested on -coo l i ng at 2300°F/1 260°C (the samp le was sec t i oned a l o n g the l o n g i ­tud ina l axis). L iqua t ion ev idence is d is ­t r i b u t e d a l o n g the g ra in b o u n d a r i e s as w e l l as w i t h i n the gra ins near the rup ­ture loca t ion . L iquat ion at elevated t e m ­perature degrades the duc t i l i t y and is a

necessary cond i t i on for base metal H A Z hot c rack ing in stainless steels.

Hot Crack Testing

Varestraint Testing

The c o n v e n t i o n a l c r i t e r i a of to ta l c rack leng th (TCL), m a x i m u m c rack length (MCL) and th resho ld strain we re adop ted for Varest ra in t test data ana ly ­sis. A n add i t i ona l c r i t e r i on , the c racked hea t -a f fec ted z o n e leng th (CHL) , p r o ­posed by L u n d i n , ef al. (Ref. 10), is also used to evaluate c rack ing sensit iv i ty and co r re la te w i t h the ho t d u c t i l i t y test re­sul ts. Figures 11 and 12 i l l us t ra te the

to ta l and m a x i m u m c rack leng th mea ­sured in the fus ion z o n e o f the c a n d i ­da te mate r ia l s s tud ied us ing the V a r e ­s t ra in t test. A t the 4 % stra in l e v e l , t he m o d i f i e d 3 1 6 mate r ia l s (bo th AX6 /CE 3890 and L316BW) show a greater max­i m u m c rack length than the Reference 316 . The m a x i m u m crack length appears to g i v e bet ter d i s c r i m i n a t i o n than the total crack length.

The w e l d metal H A Z hot c rack ing be­hav io r eva lua ted in the Vares t ra in t test (Figs. 13, 14) , shows tha t the m o d i f i e d 3 1 6 a l l oys AX6 /CE 3 8 9 0 and L 3 1 6 B W exh ib i t greater total and m a x i m u m crack lengths, w h i c h is in concer t w i t h the f u ­sion zone behav ior .

10

x \-O" Z UJ _! * (J < IT O _l < o

8 "

FUSION ZONE HOT CRACKING IN VARESTRAINT TEST

— a MODIFIED 316 (AX6/CE 3890) . . . . . » . . . , MODIFIED 316 (L316BW) - - • • - - REFERENCE316

(J < CC U

x < 2

0+»

FUSION ZONE HOT CRACKING IN VARESTRAINT TEST

— a MODIFIED 316 (AX6/CE 3890) — • • . . . . MODIFIED 316 (L316BW) - - - - - - REFERENCE316

l l l j l M M M l I | I I T I T I I I I | I I I I I I T I I J l

0 1 2 3 4 5

AUGMENTED STRAIN (%)

Fig. 11 — Fusion zone cracking behavior in the Varestraint test (total crack length vs. augmented strain).

0 1 2 3 4 5

AUGMENTED STRAIN (%)

Fig. 12 — Fusion zone cracking behavior in the Varestraint test (max­imum crack length vs. augmented strain).

1 9 4 - s I M A Y 1 9 9 3

WELD METAL HAZ HOT CRACKING IN VARESTRAINT TEST

a MODIFIED 316 (AX6/CE 3890) . . . . . . . . . MODIFIED 316 (L316BW) — * - — REFERENCE316

E E

x i -o z UJ

< cc o

o

WELD METAL HAZ HOT CRACKING IN VARESTRAINT TEST

— a MODIFIED 316 (AX6/CE 3890) . . . . . . . . . . MODIFIED 316 (L316BW) - - • • - - REFERENCE316

0.4

E — 0.3 x t -o z UJ

o < ir O

x <

0.2-

0.1

0.0

Fig. 1 (total

0 1 2 3 4 5

AUGMENTED STRAIN (%)

3 — Weld metal HAZ cracking behavior in the Varestraint test crack length vs. augmented strain).

0 1 2 3 4 5

AUGMENTED STRAIN (%)

Fig. 14 — Weld metal HAZ cracking behavior in the Varestraint test (maximum crack length vs. augmented strain).

The base metal H A Z hot crack ing sus­cept ib i l i ty measured w i t h the Varestraint test is shown in Figs. 15 and 1 6. A clear rank ing of suscept ib i l i t y to base metal H A Z hot c rack ing can be ob ta ined f rom these t w o f igures. M o d i f i e d 316 (both AX6/CE 3890 and L316BW) show a much greater tota l and m a x i m u m base metal H A Z crack length than Reference 316.

A s u m m a r y of Varest ra in t hot c rack­ing test resul ts is t a b u l a t e d in Tab le 4 . The base meta l H A Z hot c r a c k i n g sen­s i t i v i ty of the m o d i f i e d 31 6 mater ia ls is c o n s i d e r a b l y h i ghe r than the nuc lea r grade 316 stainless steels and Reference 3 1 6 . A l l the c r i te r ia (total c rack leng th , m a x i m u m c r a c k leng th and CHL) g ive the same general rank ing. The h igher base meta l H A Z ho t c rack ­ing suscept ib i l i ty of mod i f i ed 316 is be­l i eved to be d u e to the nega t i ve fe r r i te potent ia l and the fo rmat ion of l o w melt­ing eu tec t i cs o f ca rb ides and austen i te (e.g. , t i t a n i u m c a r b i d e a n d austen i te ) .

The strong tendency of the a l l oy ing e le­ments N b , Ti and V to f o r m carb ides at e leva ted t e m p e r a t u r e in the m o d i f i e d 31 6 may also be responsib le for the o b ­served higher base metal H A Z hot crack­ing suscept ib i l i ty .

F igure 1 7 shows the test z o n e o f a Vares t ra in t test samp le f r o m the m o d i ­f ied 316 (L316BW) w i t h the cracked re­gions (fusion zone , w e l d metal H A Z and base metal HAZ) marked on the macro ­g r a p h . The c rack m o r p h o l o g i e s , at h igher magn i f i ca t i on , for the three zones are shown in Figs. 1 8 - 2 0 . Intergranular-t ype c r a c k i n g is e v i d e n t . The c racks p ropaga te a l o n g c e l l u l a r d e n d r i t e boundar ies in the fusion zone and w e l d meta l H A Z and a l o n g o r i g i n a l or m i ­g ra ted g ra i n b o u n d a r i e s in the base metal HAZ.

Sigmajig Testing

S igma j ig test ing of the 316 mater ia ls was conduc ted at O R N L and the test re­

sults are documen ted in Table 5. Thresh­o ld stress is used to rank fusion zone hot c r a c k i n g suscep t ib i l i t y (if the th resho ld stress is l o w the hot c r a c k i n g suscept i ­b i l i t y is h igh) . The t h r e s h o l d stress fo r m o d i f i e d 3 1 6 (AX6/CE 3890 ) is 7 ks i . The threshold stresses for Reference 316 and m o d i f i e d 3 1 6 (L316BW) are 11 ksi and 13 ksi, respect ively. Thus, mod i f i ed 3 1 6 (L316BW) appears best a m o n g the mate r ia l s tested f r o m this s t a n d p o i n t . Therefore, the fus ion zone c rack ing sen­s i t iv i ty o f the m o d i f i e d 316 mater ia ls is s imi lar to that of the fu l l y austenit ic 316 stainless steels. In order to pe rm i t c o m ­par ison of the S igma j ig test results w i t h s im i l a r t ype mate r ia l s , the t h r e s h o l d stress values of some nuc lear grade and c o n v e n t i o n a l sta in less steels are a lso l is ted in Tab le 5. F igure 21 shows the var ia t ion in the threshold stress w i t h P+S content . The 3 1 6 N G and 3 4 7 N G al loys w i t h a p r i m a r y fe r r i t i c s o l i d i f i c a t i o n m o d e possess l o w e r sens i t i v i t y to ho t

Table 4 — Results of Varestraint Tests

MCL<J> (mm) TCL|b) (mm) Threshold strain (%)

Materials

Modified 316 (AX6/CE 3890) Modified 316 (L316 BW) Reference 316 316NC-1 316NG-2

W M HAZ

0.356 0.308 0.264 0.042 0.084

Fusion Zone

2.240 1.287 1.468 0.150 0.539

BM HAZ

0.223 0.285 0.074 0.087 0.084

W M H A Z

1.406 0.734 0.811 0.150 0.290

Fusion Zone

5.989 9.026 8.436 1.014 3.685

BMHAZ

0.789 1.377 0.263 0.305 0.219

W M H A Z

0.25 1 I

2 0.5

Fusion Zone

0-0.25 0-0.25 0-0.25

1 0-0.25

BM HAZ

0.25 1 2 1 1

CHL« (mm)

2.30 2.54 1.54 0.10 0.12

(a) Maximum crack length at 4°o strain level (b) Total crack length at 4°.. strain level (c) Cracked HAZ length at 1% strain level

WELDING RESEARCH SUPPLEMENT I 195-s

1.5

O z LU

o < LT CJ

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0.5-

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BASE METAL HAZ HOT CRACKING IN VARESTRAINT TEST

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F/g. 15 — Base metal HAZ cracking behavior in the Varestraint test

(total crack length vs. augmented strain).

a z UJ _J

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BASE METAL HAZ HOT CRACKING IN VARESTRAINT TEST

n MODIFIED 316 (AX6/CE 3890) - - • • — • MODIFIED 316 (L316BW) - - - - - - REFERENCE316

0 1 2 3 4 5 AUGMENTED STRAIN (%)

Fig. 16 — Base metal HAZ cracking behavior in the Varestraint test (maximum crack length vs. augmented strain).

c r a c k i n g as c o m p a r e d to 31 6 stainless steels w i t h a p r imary austeni t ic so l i d i f i ­cat ion mode . A l l three stainless steels in this s tudy and one f r o m pub l i shed data (Ref. 10) w i t h a p r ima ry austen i t ic so l i ­d i f i c a t i o n m o d e ( fu l ly austen i t ic ) s h o w a s im i la r l i near decrease in the th resh ­o l d stress w i t h increase in P+S conten t . In genera l , the thresho ld stresses for the mater ia ls in th is p rog ram are cons ider ­ab ly l ower than those for nuc lear grade austenit ic stainless steels. Therefore, the fus ion z o n e ho t c r a c k i n g suscep t ib i l i t y for the cand idate materials is higher than that of the nuc lear grade austenit ic stain­less steels. It shou ld be po in ted out here that B also has a s igni f icant in f luence on the hot c rack i ng resistance o f mater ia ls

(Refs. 14 -1 6). However , the effect of P+S on the hot c rack ing resistance of the ma­ter ia ls s tud ied is greater than B. This is d u e to no t o n l y the d i f f e ren t a f fec t i ng rank ing to hot c rack ing resistance (Refs. 14 -16 ) but also the di f ferent content lev­els in these materials.

Comparison of Test Results

Comparison between Cleeble and Varestraint Test Results

It is w e l l k n o w n that the hot duc t i l i t y behav io r o f a mater ia l at e leva ted t e m ­pera tu re is re la ted to base meta l H A Z hot c r a c k i n g b e h a v i o r . H o w e v e r , the conven t iona l cr i ter ia app l ied to G leeb le

Fig. 17 — Test zone of

Varestraint sample.

hot d u c t i l i t y test eva lua t ions canno t be accura te l y co r re la ted w i t h the c o n v e n ­t i ona l c r i t e r i a for Vares t ra in t H A Z ho t c rack assessment. It can be seen f r o m Tables 3 and 4 that the base metal H A Z hot c r a c k i n g sens i t i v i t y r a n k i n g p re ­d i c t ed by the D R R - 1 , DRR-2 and N D R do not co r re la te w i t h that p red i c ted by the Vares t ra in t test c r i t e r i a . H o w e v e r , the base meta l H A Z hot c rack ing sensi­t iv i ty rank ing using the RDR cr i te r ion is best corre lated w i t h TCL, M C L and C H L Varestraint test cr i ter ia. The relat ionships b e t w e e n these parameters are g i ven in Fig. 22 . As s h o w n in Fig. 2 2 , Reference 316 has the greatest RDR and the smal l ­est base meta l M C L , TCL and C H L v a l ­ues. The re fo re , t he Reference 3 1 6 has the lowest base metal H A Z hot c rack ing s u s c e p t i b i l i t y a m o n g the th ree m a t e r i ­als. The mod i f i ed 316 (L31 6BW) has the smal les t RDR, h ighest M C L , TCL a n d C H L and thus the greatest base meta l H A Z c rack ing suscept ib i l i t y . The m o d i ­f i ed 3 1 6 (AX6/CE 3890 ) shows s l igh t l y lower base metal H A Z hot c rack ing sus­cept ib i l i t y as compared to mod i f i ed 316 (L316BW).

Comparison of Varestraint and SJRmajis Test Results

The fus ion z o n e hot c r a c k i n g sensi ­t i v i t y for the mate r ia l s e v a l u a t e d w i t h bo th the Vares t ra in t and S igma j ig tests show a s imi la r t r end . The m o d i f i e d 316 stain less steels have f us i on z o n e hot c r a c k i n g suscep t i b i l i t i es s im i l a r to t he

1 9 6 - s I M A Y 1 9 9 3

RP'-* 18T

F/g. 78 — Fusion zone hot cracking morphology in modified 316 (L3I6BW).

Fig. 19 — Weld metal HAZ hot cracking morphology in modified 316 (L316BW).

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F/g. 20 — Base metal HAZ hot cracking morphology in modified 316 (L316BW).

WELDING RESEARCH SUPPLEMENT I 197-s

Table 5 — Results of Sigmajig Tests

Materials

Modified 316 (AX6/CE 3890) Modified 316 (L316 BW) Reference 316 316NC-1 316NG-2 347-1

P+S (%)

0.050 0.032 0.043 0.022 0.022 0.039

Cr Ni

0.85 0.91 1.28 1.51 1.36 1.83

Threshold Stress (ksi)

7 13 11 40 18 15

FN<a>

< 0 < 0 <0

4.2 <0

3

(a) Ferrite potential calculated by DeLong method

conventional fully austenitic Reference 316 material. However, the ful ly austenitic materials are more sensitive than the ferrite-containing 31 6 alloys. The relationship between the maximum crack length in the Varestraint test and the threshold stress in the Sigmajig test is shown in Fig. 23. As the threshold stress in the Sigmajig Test decreases the maximum crack length in the Varestraint test increases. No correlation between the threshold stress in the Sigmajig test and the total crack length in the Vare­straint test was found.

Comparison of the Hot Cracking Sensitivity between the Materials

The comparison of the hot cracking sensitivities between the different types of 316 stainless steel for fusion zone, weld metal HAZ and base metal HAZ are illustrated in Figs. 24, 25 and 26, re­spectively, using the Varestraint test re­sults. The modified 31 6 stainless steels show a slightly higher fusion zone and

weld metal HAZ hot cracking sensitiv­ity compared to the conventional fully austenitic 316 and AISI 347 stainless steels. The modified 316 stainless steels show a considerably higher base metal HAZ hot cracking sensitivity than the positive ferrite potential conventional and nuclear grade stainless steels.

Conclusions

1) The modif ied 316 stainless steel alloy possesses a strong negative ferrite potential and thus relatively higher seg­regation tendency. Titanium carbide particles were observed in base metal HAZ and minor alloying elements were found to be enriched in the dendrite cell boundaries in the fusion zone and the HAZ grain boundaries. Therefore, the hot cracking sensitivity of modified 31 6 material is predicted to be similar to the conventional fully austenitic 31 6 mate­rials and higher than that of the conven­tional 316 materials that have a primary ferritic solidification mode.

2) Hot ductil ity recovery at elevated temperature for modified 31 6 stainless steel is considerably lower than for the conventional ful ly austenitic stainless steels. Therefore, the base metal HAZ of modif ied 31 6 is predicted to exhibit a higher hot cracking sensitivity.

3) The new criterion, RDR, reflects the ducti l i ty recovery behavior in the temperature range from the ZDT to the ductility decrease temperature, and can be correlated with base metal hot crack initiation and the propagation tempera­ture range since the ducti l i ty decrease temperature is considered to represent the onset of liquation.

4) The Varestraint and Sigmajig hot cracking test results indicate that modi­fied 316 stainless steel shows a slightly higher fusion zone and weld metal HAZ hot cracking sensitivity compared to the conventional fully austenitic 316 stain­less steels.

5) The maximum crack length crite­rion for the Varestraint test and thresh­old stress criterion for the Sigmajig test are well correlated. Both can be em­ployed in evaluating fusion zone hot cracking susceptibility.

6) The new Gleeble hot ductility cri­terion, RDR, shows a very good corre­lation with the Varestraint hot cracking test criteria MCL, TCL and CHL. The RDR criterion can be used to predict base metal HAZ hot cracking behavior.

7) The practical implication of the current results is that the hot cracking potential of the modif ied 31 6 material is higher than that exhibited by current modern commercial practice 316 and 347 stainless steels of nuclear quality and is equivalent to commercial AISI

.

" * N

• 316 (PRIMARY AUSTENITIC

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MCL, TCL AND CHL (mm)

Fig. 21 — Variation in Sigmajig threshold stress with (P+S) content in different stainless steel alloys.

Fig. 22 — The relationship between the Gleeble hot ductility crite­rion RDR and the base metal Varestraint criteria MCL, TCL and CHL.

198-s I M A Y 1993

Fig. 24 — Comparison of Varestraint fusion

zone hot cracking sen­sitivity of different 316-type stainless

steels.

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THRESHOLD STRESS IN SIGMAJIG (Ksi)

Fig. 23 — The relationship between the fusion zone maximum crack length in the Varestraint test and the threshold stress in the Sigmajig test. AUGMENTED STRAIN (%)

E E. x h-O z LU - I

o < cc o s

x <

AUGMENTED STRAIN (%) F/g. 25 — Comparison of Varestraint weld metal HAZ hot cracking sensitivity of different 316-type stainless steels.

AUGMENTED STRAIN (%) Fig. 26 — Comparison of Varestraint base metal HAZ hot cracking sensitivity of different 316-type stainless steels.

WFI DING RESEARCH SUPPLEMENT I 199-s

3 4 7 . Th is resul t is no t u n e x p e c t e d be­cause o f t he f u l l y aus ten i t i c s o l i d i f i c a ­t i on m o d e of the m o d i f i e d 3 1 6 ma te r i ­als (negat ive fe r r i te po ten t i a l ) and the m ino r e lement add i t ions.

Acknowledgment

The authors a c k n o w l e d g e the f i n a n ­c ia l suppo r t by the U.S. D e p a r t m e n t o f Energy, t h rough Fossil Energy Mater ia ls P rog ram, ope ra ted by M a r t i n M a r i e t t a Energy Systems, Inc., at O a k Ridge N a ­t ional Laboratory.

References

1. Swindeman, R. W., Goodwin, C. M., Maziasz, P. J., Judkins R. R., and Devan, J. H. 1986. Alloy design criteria and evaluation methods for advanced austenitic alloys in steam service, ORNL-6274.

2. Swindeman, R. W., Goodwin, G. M., and Maziasz, P. J. 1987. Procurement and screening test data for advanced austenitic alloys for 650°C steam service (Part 1, 1 4Cr-16Ni steels and 20Cr-30Ni-Fe alloys), ORNL/TM-10206/P1.

3. Carolan, R. A., Li, C. Y., Maziasz, P. J., Swindeman, R. W., and Todd,). A. 1987. The strengthening mechanisms in modified AISI 316 stainless steels, ORNL/SUB/85-

27488/01. 4. Black, R. D „ Carolan, R. A., Li, C. Y.,

Swindeman, R. W „ and Maziasz, P. ). 1989. Effect of thermal-mechanical pretreatments on the f low strength of a copper modif ied, stabilized 316 stainless steel. ORNL/SUB/85-27488/03.

5. Todd, ). A. 1 988. Microstructure stud­ies of advanced austenitic steels. Semiannual progress report. ORNL/FMP-88/2. pp. 329-350.

6. Lundin, C. D., Qiao, C. Y. P., and Lee, C. H. 1990. Standardization of Gleeble hot ductility testing — part I I : experimental eval­uation. Proceedings of Conference on Weld­ability of Materials, pp. 9-22.

7. Lundin, C. D., Menon, R., Osorio, V., and Lee, C. H. 1985. New concepts in Vare­straint testing for hot cracking. Welding Re­search the State of Art, Proceedings of the JDC University Symposium. Editors: E. Nippes and D. J. Ball, International Welding Congress, ASM, Toronto, Canada.

8. Goodwin, G.M. 1987. Development of anew hot cracking test — the Sigmajig. Weld­ing Journal 66(2): 33-s to 38-s.

9. Yeniscavich, W. 1969. Correlation of hot duct i l i ty curves wi th cracking during welding. Proceeding of Symposium on Meth­ods of High Alloy Weldability Evaluation, WRC, New York, N.Y.

10. Lundin, C. D., Lee, C. Pi., and Qiao, C. Y. P. 1988. Final report of the group-spon­sored study on weldabil i ty and hot ducti l i ty

behavior of nuclear grade austenitic stainless steels. Weld ing Research and Engineering Group, The University of Tennessee.

11. Swindeman, R. W., Goodwin, G. M., Maziasz, P. J., and Boiling, E. 1988. Procure­ment and screening test data for advanced austenitic alloys for 650°C steam service, (part 2, final report), ORNL/TM-1 0206/P2.

12. Lundin, C. D., Qiao, C. Y. P., Kikuchi, Y., Shi, C , and Gi l l , T. P. S. 1991. Investiga­tion of jo in ing techniques for advanced al ­loys. DOE Report, ORNL/Sub/88-07585/02.

1 3. Maziasz, P. J. 1 989. Developing an austenitic stainless steel for improved perfor­mance in advanced fossil power facil it ies. Journal of Metals, pp. 14-20.

14. Canonico, D. A., Savage, W. F., Werner, W. J., and Goodwin , G. M. 1969. Effects of minor additions on weldabi l i ty of Incoloy 800. Conference Proceedings of WRC Symposium on Effects on Minor Ele­ments on the Weldability of High Nickel Al­loys, pp. 68-92.

15. Brooks, |. A. 1974. Effects of al loy modif ications on HAZ cracking of A-286 stainless steel. Welding Journal 53(11): 51 7-s to 523-s.

1 6. Hu l l , F. C. 1 960. Effects of al loying additions on hot cracking of austentic chromium-nickel stainless steels. American Society for Testing and Materials, Vo l . 60, pp. 667-690.

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