Atom-probe investigation of precipitation in 12% Cr steel weld metals

apphed surface sctence ELSEVIER

Applied Surface Science 76/77 (1994) 248-254

Atom-probe investigation of precipitation in 12% Cr steel weld metals G u a n g - J u n Cal a,,, Lars L u n d l n

a,

H a n s - O l o f A n d r d n d, Lars-Erlk Svensson b

" Department of Physics, Chalmers Umversay of Technology, S-41296 Goteborg, Sweden h Central Laboratories, The Esab Group, P 0 Box 8004, S-40277 Goteborg, Sweden

(Recewed 2 August 1993, accepted for pubhcation 31 August 1993)

Abstract The mlcrostructure of two types of 12% Cr steel weld metals, one with the composition of a common 12% Cr steel and the other with a higher nitrogen content, was studied using TEM (transmission electron microscopy) and A PF I M (atom-probe field-Ion microscopy) in post-weld heat-treated condmon The mlcrostructure of the 12% Cr weld metals consisted of tempered martenslte, retained 8-ferrlte, an irregular low-dislocation c~-fernte and precipitates Precipitates m the weld metals were dominantly M23C 6 on different boundaries Plate-hke and free cubic MN and M2N were found inside the a-ferrlte AP F I M analysis showed that M23C 6 was almost a pure carbide and MN was almost d pure mtnde Carbon and nitrogen in the weld metals mainly existed in the precipitates High nitrogen content did not change the composlUon of the precipitates, but increased the quanhty of nltrldes Therefore, in the high nitrogen weld metal, the content of strong nltnde-formmg elements m the matrix decreased These results are important In order to understand the strengthening mechanism of the high Cr steel weld metals, as well as of other high Cr heat-resistant steels

1. Introduction

Because of their high creep strength and hot corrosion resistance, 12% Cr steels are widely used In the power and chemical lndustrtes as heat-resistant steels The composition of 12% Cr steels is based on chromium and molybdenum The steels also contain some W, V, N1 and other alloying elements The carblde-formmg elements, Cr, Mo, V, etc, react with carbon to form different carbides Many types of carbides, mcludlng M~C, M23C 6, MvC 3, M2C and MC, have been

* Corresponding author Fax +46 31 165176

reported in 12% Cr steels [1-6] It ts these carbides that strengthen the material at high temperatures Identification of the precipitates and study of the precipitation process have, therefore, been an important part of the studies of 12% Cr steels In recent years, nitrogen has been used as an alloymg element to substitute part of the carbon in the steel It is claimed that some benefits could be obtained from nitrogen in the steels [7-10] StmIlar to carbon, nitrogen is an interstitial element, and can form nitrldes with the carbide-forming elements It is beheved that in nitrogen-containing steels, the carbide precipitates would be replaced by the complex carbomtrldes So, in the literature, carbon and nitrogen in for-

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249

alloying element in high Cr steels, It ms perhaps more attractive to use nitrogen m the weld metal In 12% Cr steel weld metals, because of the effect of high Cr content, some 6-ferrlte may be retained to room t e m p e r a t u r e The retained 6ferrlte is considered to be detrimental to the properties of the 12% Cr steels, by reducing the maximum strength and Impact toughness [17] Nitrogen is an austentte-formmg element, and tt may be used to decrease the quantity of retained 6-ferrlte without harming the weldablhty of the material The behavlour of high nitrogen 12% Cr steel weld metals, including the development of mtcrostructure and mechanical properttes, has not yet been systemattcally studied In order to understand the effect of nitrogen on the mlcrostructure and mechanical properties of 12% Cr steel weld metals, we studied the mlcrostructure (especially the precipitates) in a common and in a high nitrogen 12% Cr steel weld metal A P F I M and T E M (transmission electron microscopy) were used for this study The results of the investigation are presented in this p a p e r

mulae of prectpltates are denoted as X, to stand for carbon + nitrogen However, the effect of nitrogen on the preclpttation and the development of mlcrostructure in the high Cr steels is not completely understood The reason for this is partly because of mlcroanalysts difficulties, it has been dtfficult to separate and analyse light elements (as nitrogen and carbon) in precipitates as well as in the matrix At this point, however, atom-probe field-lon mtcroscopy (APFIM) is a powerful method, and it could be used to solve these problems As 12% Cr steels are being more widely used, their welding 1s becommg more important The behavlour of a weldment depends on the parent metal and weld metal Therefore, tt is clearly necessary to study the weld metal in order to match the development of the weld metal with that of the parent metal As in the case of the parent material, the mtcrostructure and mechanical properties are the main subjects in the study of 12% Cr weld metals [11-16] Because of the nature of welding, transformation conditions and, consequently, the mtcrostructure of the weld metal differ trom the common forged and heattreated parent 12% Cr steels The mlcrostructure of a steel depends on its primary state and heat treatment To weld the 12% Cr steels, preheating and post-weld heat treatment ts necessary Before post-weld heat treatment, tf the weldment is cooled enough so that austenIte in the weld metal has transformed to martensite, the post-weld heat treatment will be the process of martenstte tempering Otherwise, if some austenite is retained untransformed before heat treatment, the retained austenlte will decompose during heat treatment or subsequent cooling Nitrogen IS a universal element in weld metals As nitrogen is being more and more used as an

2. Experimental 2 1 Materials The materials studied were weld deposits of two different 12% Cr steel electrodes on a base plain carbon steel The base matertal had a thickness of 20 m m and the jomt geometry was compatible with ISO 2560 Multlpass manual metal arc welding was employed, using electrodes 3 25 m m in diameter and wtth as welding parameters current 120 A, voltage 22 V, welding speed 4 m m / s , preheat and interpass t e m p e r a t u r e 300°C After welding, the weldment was heat-treated

Table 1 Chemical composition of the weld metals (wt%) Material

Cr

Mo

W

V

Nl

C

N

Fe

Mn

St

Others

C1 C3

10 86 11 01

0 97 1 44

0 50 0 69

0 33 0 53

0 72 0 69

0 19 0 23

0 01 0 098

84 95 83 13

0 77 0 86

0 60 0 62

Bal Bal

C1 the common 12% Cr steel weld metal C3 the high mtrogen 12% Cr steel weld metal

250

G -J Cat et al /Apphed Surface Sctence 76/77 (1994) 248-254

immediately at 750°C for 5 hours, then cooled in the furnace to room t e m p e r a t u r e The chemical composition of the weld metals was m e a s u r e d by optical emission technique and the result is gwen in Table 1 One of the two weld metals had the composition of a common 12% Cr steel, and the other had a high nitrogen content 22

men t e m p e r a t u r e of 50-60 K, with a residual gas pressure below 4 × 10-~0 mbar and a pulse fraction of 20% The pulse repetition frequency was 200-267 Hz, resulting in a collection rate of 5 0 100 ions per rnlnute Neon was used as image gas, with a partial pressure of 4 × 10 -4 mbar, and at a specimen t e m p e r a t u r e of 50-60 K

TEM

3. Results and discussion Microstructure of the weld metals was first investigated with a J E O L 2000FX 200 kV T E M / S T E M The precipitates were identified by the selected area electron diffraction ( S A E D ) m e t h o d The thin foil specimens for T E M analysis were first cut from the weld metals as slices of 0 3 mm in thickness The shces were mechanically ground to a thickness of about 0 1 mm and then e l e c t r o p o h s h e d to the desired thickness for T E M in a 10% solution of perchloric acid m ethanol at a t e m p e r a t u r e < - 30°C and at a voltage of 25 V 2 3 APFIM A P F I M was employed to analyse the composition of matrix and precipitates in the weld metals The a t o m - p r o b e instrument, and the methods used for corrections during analysis and evaluation, have all b e e n described previously [18-21] In short, the Instrument is an energy-comp e n s a t e d t i m e - o f - f l i g h t a t o m p r o b e with a chevron-mounted multi-channel plate detector To p r e p a r e the A P F I M specimens, the weld metals were first cut to pins of about 0 3 × 0 3 mm cross section Then a neck was m a d e on the pins by electropohshlng at 20 V and room t e m p e r a ture in a thin layer of an electrolyte consisting of 10% perchloric acid, 15% glycerol and 75% ethanol floating on t n c h l o r o e t h y l e n e The neck was further electrolytically thinned in a 2% solution of p e r c h l o n c acid in 2-butoxyethanol at 15 V and room t e m p e r a t u r e , until it broke The tips were studied by T E M prior to A P F I M analysis, in o r d e r to identify the m~crostructure of the specimens A specimen generally had to be backp o h s h e d several times m o r d e r to make the required microstructure accessible for A P F I M analys~s Analyses were p e r f o r m e d m U H V at a specl-

3 1 General mtcrostructure o f the weld metals The microstructure of the weld metals was identified by T E M and it consisted of t e m p e r e d martenslte, some r e t a i n e d ~-ferrlte, an Irregular low-dislocation a - f e r r i t e and precipitates The most p r o m i n e n t precipitates were M23C 6 They were generally coarse particles on the retained 6-ferrlte and martensltlC lath boundaries (Fig 1) Inside the irregular low-dislocation a - f e r r i t e two types of precipitates were found O n e type of the precipitates was fine and cubic-like (about 20 × 20 nm), the other was relatively coarse and elliptical plate-hke (about 70 × 45 nm) (Fig 2) By indexing the S A E D patterns, it was found that two types of mtrldes were present MX and M 2 X MX had a fcc lattice structure with the lattice p a r a m e t e r a = 0 414 nm M 2 X had an hcp lattice structure with the lattice p a r a m e t e r s a - - - 0 283 nm and

Ftg 1 TEM m~crograph of typical coarse M23C6 on a retamed ~3-fernte boundary m the high mtrogen weld metal Bright-field image

G -J Cat et al/Apphed Surface Scwnce 76/77 (1994) 248-254

251

c ~ 0 454 nm Both MX and MEX could have the fine cubic or coarse elhptlcal plate morphologies Comparing the common and high nitrogen 12% Cr weld metals, the size of MX and M2X was identical, but the density of the precipitates, especially of the fine cubic precipitates, was higher in the high nitrogen weld metal than in the common 12% Cr steel weld metal

3 2 APFIM analysts of the weld metals 3 2 1 Composttton ofprectpttates m the weld metal Fig 3 shows field-ion mlcrographs of MX precipitates A coarse preczpltate shows a plate-like morphology in the field-Ion image (the precipitate in the image may be smaller than the size of

Fig 3 Field-Ion mlcrographs of the weld metals As indicated by the arrows on the mlcrographs (a) fine precipitate and (b) coarse plate-like precipitate in the high mtrogen 12% Cr weld metal In (a) the voltage was 6 2 kV and the tlp-to-screen distance was 5 cm The diameter of the Image is about 50 nm In (b) the voltage was 7 3 kV and the tip-to-screen distance was 6 cm The diameter of the image is about 60 nm

Fig 2 TEM mlcrographs of (a) fine precipitates inside the low-dislocation ferrlte m the common 12% Cr weld metal, (b) coarse plate-hke precipitates reside the low-dislocation ferrlte m the high nitrogen 12% Cr weld metal Bright-field images

a whole particle because It was only part of it) Typical compositions of precipitates in the weld metals are given in Table 2 and Table 3 It can be seen that M23C 6 was chromium rich, containing also iron and molybdenum Some vanadium and tungsten had dissolved i n M 2 3 C 6 The ratio of

G -J Cat et al /Apphed Surface Science 7 6 / 7 7 (1994) 248-254

252

Table 2 C o m p o s m o n o f m a t r i x a n d p r e c i p i t a t e s m m a t e r m l C1 Matrix (wt%)

Fe Cr Mo V Nb W C N Nl Mn Sl P

879 +05 311-+023 210 -+06 755 _+026 7 9 7 - + 0 3 8 4 9 8 _+08 149 -+016 011_+004 407-+024 015 -+004 479 _+12 107_+012 0 6 6 + 0 09 052 _+013 066-+010 0008_+0004 082_+010 220 +14 3 9 4 _+1 3 078 -+009 019_+005 063 -+008 120_+013 092 _+007 ~ 003+002 002 -+001 -

Total no of 10164 atoms m analysis

MN (at%)

M 23C 6 (at%)

Element

7400

6837

In t h e m a t r i x , a low q u a n t i t y o f m t r o g e n m a y b e i n c l u d e d in this v a l u e

metallic content to carbon, M C, was somewhat lower than the stoichlometrlc ratio of 23 6 MX precipitates, both the fine and coarse, were vanadium-rich nltrides (so, hereinafter, we will denote the MX as MN) Besides the dominating vanadium component, other metallic elements, mainly Cr and some Fe had dissolved in MN Nb, which was a trace element in the weld metal, clearly

Table 3 C o m p o s i t i o n o f m a t r i x a n d p r e c i p i t a t e s in m a t e r i a l C3 Element

Matrix (wt%)

MN (at%)

Fe Cr Mo V Nb W C N NI Mn Sl

870 +06 886_+029 1 1 0 - + 0 14 007-+002 048_+013 -

066_+012 207 +06 8 7 4 _ + 0 5 1 4 8 2 _+09 4 22+025 4 9 5 -+1 7 126-+02(} 1 05 _+ 0 15 061+010 066+012 236+12 394 +17 0 36 + 0 08 0 9 4 + 0 12 002-+002

p

T o t a l no o f a t o m s in a n a l y s i s

0 83 + 0 09 0 8 6 + 0 10 078_+006 ' -

9552

M 23C~, (at%)

_

4840

6368

' In t h e m a t r i x , a low q u a n t i t y o f m t r o g e n m a y b e i n c l u d e d m this v a l u e

enriched in MN According to the A P F I M analysis, the nitrogen content (with the small carbon amount added) in MN was below that given by the stolchlometrlc ratio of 1 1 The composition of the precipitates vaned to some extent from particle to particle This is because the alloying elements distributed heterogeneously in the matrix and the transformation conditions of the precipitates were different Considering the variation in composition in different particles, it could be concluded that the compositions of the precipitates in the two weld metals were identical It is noticeable that no nitrogen dissolved in M > C 6 , while MN was almost free of carbon, though nitrogen atoms are lnterstltlals in the precipitates just as carbon Generally it is believed that carbon and nitrogen could substitute each other in a precipitate Unfortunately, perhaps because of the low density, we were not able to analyse M2X by A P F I M However, considering the morphology, distribution and EDS analysis of the precipitates [16], it can be expected that M z X w a s also a vanadium-rich nltrlde in these steels

3 2 2 Composmon of matrix Typical matrix compositions of the weld metals are also given in Table 2 and Table 3 The composttlon of the weld metals was not homogeneous and changed from place to place In most cases, the content of Cr, Mo, V and other carbonltrlde formers was lower than the chemical composition of the materials The carbon content in the matrices was very low Considering the detection limit of the method, the matrtx of both materials could be considered to be almost free of carbon The reduction in the contents of carbon- and carbide-forming elements m the matrix is due to of the precipitation of the second phases In the common 12% Cr steel weld metal, some carbon-segregated areas could be found In such areas, the carbon content was very high, and the Cr content was also higher than the chemical composition of the material In the matrix analysis, the detected SI and N1 contents were generally higher than in the chemical composition of the weld metals The higher the analysis temperature, the more Sl was detected This was probably

G -J Cat et al/Apphed Surface Sctence 76/77 (1994) 248-254 d u e to p r e f e r e n t i a l e v a p o r a t i o n I n t h e m a s s - t o c h a r g e s p e c t r a o f m a t r i c e s , SI 2+ a n d N + p e a k s a r e n o t d i s t l n g u t s h a b l e Since no N 2+ ions w e r e d e t e c t e d , a n d t h e a m o u n t s of 28512+, 29512+ a n d 3°SI2+ a g r e e d with t h e n a t u r a l a b u n d a n c e , we c a n c o n c l u d e t h a t t h e p e a k at 14 in t h e m a s s - t o - c h a r g e s p e c t r u m was m a i n l y f r o m sihcon, a n d t h e m a t r i x in m o s t a r e a s was a l m o s t free o f n i t r o g e n This m e a n s that, hke c a r b o n , m o s t o f t h e n i t r o g e n in the w e l d m e t a l s e x i s t e d in the p r e c i p i t a t e s

3 2 3 The effect of mtrogen on the preclpltatton Comparing compositions of the precipitates a n d m a t r i c e s o f the two w e l d m e t a l s (see T a b l e 2 a n d T a b l e 3), t h e c o m p o s i t i o n o f t h e p r e c i p i t a t e s in the two w e l d m e t a l s was a l m o s t i d e n t i c a l T h e c o n t e n t o f v a n a d i u m m the high n i t r o g e n w e l d m e t a l m a t r i x was s o m e w h a t l o w e r t h a n in t h e c o m m o n 12% Cr steel w e l d m e t a l This IS b e cause during heat treatment, nitrogen reacted with the v a n a d i u m a n d p r e c i p i t a t e d o u t as M N a n d M 2 N T h e m o r e n i t r o g e n in t h e weld m e t a l , t h e m o r e p r e c i p i t a t e s c o u l d form, a n d it r e s u l t e d in a l o w e r c o n t e n t o f v a n a d i u m a n d o t h e r n l t r t d e - f o r m l n g e l e m e n t s This IS c o n s i s t e n t with T E M o b s e r v a t i o n It can b e c o n c l u d e d t h a t nitrogen in t h e w e l d m e t a l i n c r e a s e s the q u a n t i t y o f t h e p r e c i p i t a t e s b u t d o e s not affect t h e i r c o m p o sition A s the n i t r o g e n c o n t e n t in t h e weld m e t a l Increases, the c o n t e n t o f s t r o n g n l t r i d e - f o r m i n g e l e m e n t s , like v a n a d i u m , will d e c r e a s e In t h e matrix

4. Conclusions A c c o r d i n g to t h e T E M a n d A P F I M lnvestlgatlon o f c o m m o n a n d high n i t r o g e n 12% C r steel w e l d metals, we can c o n c l u d e • The mlcrostructure of both the common and t h e high n i t r o g e n 12% Cr steel w e l d m e t a l consisted o f t e m p e r e d m a r t e n s l t e , r e t a i n e d ~-ferrlte, an i r r e g u l a r l o w - d i s l o c a t i o n a - f e r r l t e a n d p r e c i p i tates • T h e p r e c i p i t a t e s in b o t h t h e c o m m o n a n d high n t t r o g e n 12% Cr steel w e l d m e t a l s w e r e p r e d o m i n a n t l y M23C 6 P l a t e - l i k e a n d fine cubic M N a n d

253

M 2 N w e r e f o u n d to have p r e c i p i t a t e d in t h e irregular low-dislocation a-ferrtte • C a r b o n a n d n i t r o g e n in t h e w e l d m e t a l s existed in t h e p r e c i p i t a t e s T h e t e m p e r e d martensltiC matrices w e r e a l m o s t free o f c a r b o n a n d n i t r o g e n • M23C 6 was a l m o s t a p u r e c a r b i d e a n d M N was a l m o s t a p u r e n t t r i d e N c o n t e n t in M N was lower t h a n t h a t given by t h e Stolchlometrical r a t i o o f

1 1 • A high n i t r o g e n c o n t e n t in t h e 12% Cr steel w e l d m e t a l s did n o t c h a n g e t h e c o m p o s i t i o n o f the prectpltates, but increased the quanttty of n l t r l d e p r e c t p l t a t e s T h e c o n t e n t o f strong ntt r i d e - f o r m l n g e l e m e n t s in the m a t r i x t h e r e f o r e decreased

5. Acknowledgements F i n a n c i a l s u p p o r t f r o m the E s a b G r o u p a n d t h e S w e d i s h N a t i o n a l B o a r d for I n d u s t r i a l a n d T e c h n i c a l D e v e l o p m e n t ( N U T E K ) is g r a t e f u l l y acknowledged

6. References [1] Kehsln Kuo, J Iron Steel Inst 174 (1953) 363 [2] K J Irvlne, D J Crowe and F B Plckermg, J Iron Steel Inst 195 (1960) 386 [3] A Hede and B Aronsson, J Iron Steel Inst 207 (1969) 1241 [4] J M Vitek and R L Klueh, Met Trans 14A (1983) 1047 [5] R L Klueh and PJ Mamasz, Met Trans 20A (1989) 373 [6] RC Thomson and H K D H Bhadeshla, Met Trans 23A (1992) 1171 [7] P J Uggowltzer, B Anthamatten, M O Spmdel and G Stem, m Advances m Material Technology for Fossd Power Plants (ASM, Cleveland, OH, 1987) p 181 [8] F B Plckermg, m High Nitrogen Steels '88 (The lnshtute of Metals, London, 1989) p 10 [9] BR Anthamatten, PJ Uggowltzer, ML Cul, MO Speldel and G Stem, in High Nitrogen Steels '88 (The Institute of Metals, London, 1989) p 58 [10] H Berns and F Krafft, m High Nitrogen Steels '88 (The Institute of Metals, London, 1989) p 169 [11] T Lechtenberg, m Ferntlc Steels for High-Temperature Apphcahons (ASM, Cleveland, OH, 1983) p 163 [12] C S Wright and T N Baker, m Stainless Steels '87 (The Institute of Metals, London, 1988) p 415

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[13] C Coussement, M de Wltte, A Dhooge, R Dobbelaere, M Steen, E van der Donckt and L van Muysen, Rev Soudure 3/4 (1988) 36 [14] S M Paganl and F P A Robinson, Mater Scl Tech 4 (1988) 554 [15] G-J Cal, H - O Andr6n and L - E Svensson, m The 3rd Int Conf on Trends m Welding Research (ASM, Cleveland, OH, 1993) p 581 [16] G - J Cal, H - O Andr~n and L - E Svensson, Investigation of m~crostructure development m a 12% Cr Steel weld metal containing h~gh mtrogen content, in Proc 3rd Int Conf on High Nitrogen Steels (HNS-93), Klev, Ukraine, 14-16 Sept 1993, to be pubhshed

[17] J Z Bnggs and T D Parker, The Super 12% Cr Steel (Chmax Molybdenum Company, New York, 1965) [18] H - O Andr6n and H Nord~n, Scand J Metall 8 (1979) 147 [19] H -O Andr6n, J Phys (Pans) 47 (1986) C7-483 [20] U Rolander, Atom-Probe Mlcroanalysls of Cermets, Thesis, Chalmers Umverslty of Technology, Goteborg, 1991 [21] L Lundln and U Rolander, Appl Surf Scl 67 (1993) 459