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Influence of austenitizing temperature on microstructure and mechanical properties of AISI 431 martensitic stainless steel electron beam welds
Materials and Design 30 (2009) 1612–1624
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Influence of austenitizing temperature on microstructure and mechanical properties of AISI 431 martensitic stainless steel electron beam welds A. Rajasekhar a,*, G. Madhusudhan Reddy b, T. Mohandas b, V.S.R. Murti a a b
Department of Mechanical Engineering, SVITS, Mahaboob Nagar 509 001, India Defence Metallurgical Research Laboratory, Hyderabad 500 058, India
a r t i c l e
i n f o
Article history: Received 25 May 2008 Accepted 18 July 2008 Available online 31 July 2008 Keywords: Martensitic stainless steel Electron beam welding Austenitization temperature Microstructure Mechanical properties
a b s t r a c t The relative effects of various austenitizing temperatures on microstructure and mechanical properties of electron beam welds of AISI 431martensitic stainless steel were studied. The post-weld heat treatments consist of austenitizing the weld samples for 1 h at various temperatures, i.e., at 950 °C, 1000 °C, 1050 °C, 1100 °C and at 1150 °C and air cooling followed by double tempering, i.e., tempering at 670 + 600 °C. In the as-welded condition the microstructure contains dendritic structure with ferrite network and retained austenite in a matrix of un-tempered martensite. The prior austenite grain size increased with increase in austenitizing temperature. Parent metal grain size was coarser as compared to grain size in the weld zone in respective conditions. Retained austenite content increased with increase in the austenitizing temperature. Presence of undissolved carbides was observed in welds and parent metal austenitized up to 1000 °C and they dissolved at austenitizing temperature P 1050 °C. Coarsening of martensite laths was observed after tempering. The martensite laths were coarser in the samples subjected to higher austenitizing temperatures. Optimum mechanical properties, i.e., strength, hardness and toughness were observed when austenitized between 1050 °C and 1100 °C followed by tempering. Austenitizing at 1150 °C and tempering resulted in inferior mechanical properties. The mechanism for the observed trends is discussed in relation to the microstructure, fracture features and mechanical properties. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Martensitic stainless steels containing 17Cr–2Ni are commonly used in quenched and tempered condition. The heat treatment consists of annealing to obtain austenite and dissolve the carbides, followed by cooling to transform the austenite into martensite and subsequent tempering of martensitic microstructure. Depending on the composition and processing history, the microstructure of martensitic stainless steel consists of martensite, undissolved carbides as well as reprecipitated carbides, retained austenite and dferrite. It is well-known that properties obtained in these steels are strongly influenced by such treatments. The amount of carbides in the as-quenched microstructure exerts an influence on the properties of these materials such as hardness, strength, toughness, corrosion and wear [1]. Earlier studies on 17Cr–2Ni martensitic stainless steel have revealed that due to the composition imbalance between austenite and ferrite stabilizing elements in
* Corresponding author. Tel.: +91 9989 421206; fax: +91 8542 236920. E-mail address: [email protected] (A. Rajasekhar). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.042
the steel and inappropriate austenitizing temperatures employed, the steel could end up with microstructure containing large amount of retained austenite, d-ferrite and carbides in a martensitic matrix [2,3]. The criteria for a suitable alloy design demands a low content of d-ferrite as it is deleterious for strength [4,5], toughness [6,7] and corrosion resistance [2,8] and maximum amount of carbon in solution for greater hardness and strength of martensite [9]. Some of the previous studies revealed that volume fraction of d-ferrite does not change up to an austenitizing temperature of 1100 °C, but increases with increasing austenitizing temperature above this temperature [2,9]. This observation indicates that up to <1100 °C, the 17Cr–2Ni steel remained in the c-phase field and entered into (c + d) phase field above 1100 °C. Therefore, below 1100 °C for a given composition of the steel, the strength mainly depends on the extent of dissolution of carbides and alloying elements in the austenite, which on quenching forms strong martensite. The hardness and strength of as-quenched martensite will dictate the strength levels achievable on tempering at suitable temperatures. The recommended austenitizing temperatures by various international standards based on composition and heat treatments lies anywhere between 950 °C and 1100 °C. However, these recommendations do not appear to be supported
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Table 1 Chemical composition of martensitic stainless steel type AISI 431 Element
C
Si
Mn
Cr
Ni
S
P
Weight (%)
0.17
0.6
1.0
17.0
2.0
0.03
0.04
Table 2 Welding parameters for EB welding Machine settings/parameters Gun to work distance, mm Accelerating voltage, KV Beam current, mA Focus Speed, m/min Vacuum level, mbar Heat input, J/mm
283 55 35 mA (for initial first pass for pre heating) and 65 mA (for penetration) Slightly above the surface 1.0 10 4 mbar and less 214.5
Fig. 1. Location and orientation of the Charpy V-notch impact sample in the weld zone (schematic).
by systematic experimental investigations correlating properties to microstructure. Considerable research work has been carried out
Fig. 2. Optical microstructures of parent metal in different austenitized conditions (before tempering) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
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Fig. 3. Optical microstructures of EB weld interfaces in different austenitized conditions (before tempering) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
on 17Cr–2Ni martensitic stainless steel by Balan et al. [10] in the direction of correlating microstructure with properties by varying austenitizing temperatures. The optimum austenitization temperature of the 17Cr–2Ni steel is found to lie between 1050 °C and 1100 °C w.r.t. to dissolution of carbides, minimum d-ferrite and fine grain size that resulted in the best combination of hardness, strength, ductility and notch toughness in the as-quenched condition. Since martensitic stainless steels are structural materials, weldability has been an important consideration in their development. In general, these steels are readily weldable by conventional welding processes, although special precautions are necessary to avoid
hydrogen cracking because of the low Ms temperature and the propensity of forming fine and brittle martensite even at normal air cooling rates [11]. Welding procedures are designed primarily to avoid hydrogen cracking and to obtain adequate weldment toughness [12]. To weld martensitic stainless steels both preheat and post-weld heat treatments (PWHT) are necessary to reduce the hardness of the martensitic regions thereby ensuring the structural integrity of the weld region during service. Conventional arc welding characteristics of martensitic stainless steels have been examined by several researchers [13–16]. However, data on high energy density processes such as electron beam (EB) welding are scarce. Electron beam welding is character-
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Fig. 4. Optical microstructure of EB welds in different austenitized conditions (before tempering) (a) 950 °C (b) 1000 °C (c) 1050 °C (d) 1100 °C and (e) 1150 °C.
ized by a short interaction time with intense energy density. As a result, the process has a lower heat input, which is expected to exercise greater control over grain coarsening and distortion. EB welding can offer advantage on higher joint completion rates for a variety of engineering fabrications. The influence of post-weld heat treatments on microstructure and mechanical properties of electron beam welds of AISI 431 martensitic stainless steel has been evaluated by Rajasekhar et al. [17], by employing different post-weld heat treatments, consisting of austenitizing the weld samples at 950 °C and air cooling followed by tempering at 400 °C, 600 °C, 670 °C and 670 + 600 °C. It has been reported that although 950 °C is about 50–100 °C above the upper critical temperature
(Ac3) for this steel, depending on the carbon and chromium contents, the steel might require a higher austenitization temperature for complete dissolution of chromium carbides [2,9]. However, grain coarsening, retention of austenite, and formation of d-ferrite in the as-quenched microstructure need to be taken in to consideration while selecting a suitable austenitizing temperature. Published information available regarding welding behavior of martensitic stainless steel AISI 431 is very limited. The lack of knowledge constitutes a potential drawback to the more widespread use of welded martensitic stainless steels. Keeping the above in view, it is envisaged to evaluate the influence of austenitization temperature on the microstructure
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Fig. 5. Grain size of parent metal, HAZ and welds in different austenitized conditions.
Fig. 6. SEM microstructures of parent metal and welds in as-quenched condition. (a) Parent metal at 950 °C; (b) parent metal at 1050 °C; (c) weld at 950 °C and (d) weld at 1050 °C.
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Fig. 7. Influence of austenitizing temperature on retained austenite.
and mechanical properties of 17Cr–2Ni steel welds. Parent metal was also subjected to similar treatments for comparative evaluation.
2. Experimental details The parent material used in this study is AISI 431 martensitic stainless steel whose chemical composition is given in Table 1. The material was received in 30 mm diameter rod form in fully annealed condition. It is then forged at 1200 °C to 5 mm thick plates. Forged plates were annealed at 670 °C for 1 h and air cooled prior to welding. Autogenous bead on plate full penetration electron beam welding of the plates was carried out. The welding parameters are given in Table 2. Initial low power welding has been adopted to preheat the material. Welded samples were austenitized for 1 h at selected temperatures in the range 950–1150 °C with an interval of 50 °C followed by air cooling to room temperature. The air cooled samples were subsequently double tempered, i.e., tempered at 670 °C for 2 h followed by air cooling and subsequently tempering at 600 °C for 2 h followed by air cooling. For comparison purpose parent metal has also been subjected to similar heat treatments. Mechanical properties, i.e., micro-hardness, transverse tensile strength and Charpy ‘V’-notch impact toughness of welds and parent metal in different heat treated conditions were evaluated. Tensile and Charpy V-notch specimens were made to sub-size as per ASTM standards, and were tested at room temperature. Notch is located in the centre of the weld joint. The location and orientation of the Charpy V-notch impact sample in the weld zone is shown in Fig. 1. Micro-hardness survey was carried across the joint interface at the centre on a Matsuzawa micro-hardness tester using a Knoop indenter. Standard metallographic samples were prepared and examined for microstructural details under Leitz optical and stereo microscopes after etching. Kallings reagent (cupric chloride – 1.5 g, HCl – 33 ml, ethanol – 33 ml and distilled water – 33 ml) was used to reveal the microstructure after tempering. Electrolytic etchant
(60 parts nitric acid, HNO3 and 40 parts H2O) was used to reveal the prior austenite grain boundaries. Retained austenite (cr) content in the welds and parent metal in various heat treated conditions was estimated using Stress Tech 3000 X-ray system using Cr Ka radiation. The volume fraction of austenite was estimated from measurements of the integrated intensities of martensite, ferrite and austenite peaks assuming they are the only phases present. Fractography of the tensile and impact samples was carried by employing LEO scanning electron microscope (SEM). The impact samples were sectioned in mid section perpendicular to the notch to examine crack path under an optical microscope. The prior austenite grain size of quenched parent and weld zones in various austenitized conditions were measured using linear intercept methods. At least eight intercepts across the length, breadth and diagonal of the photographs at a magnification of 100 were taken on five photographs chosen from different areas of the sample. 3. Results and discussion 3.1. Microstructure 3.1.1. As-quenched microstructure Optical metallography was carried out in the as-quenched condition to identify the variation in the gross features of microstructure, e.g. prior austenite grain size, retained austenite content and undissolved carbides of the parent metal and welds austenitized at 950 °C, 1000 °C, 1050 °C, 1100 °C and 1150 °C. Typical optical microstructures at the fusion boundary, in the parent metal and weld zone in different austenitized conditions are shown in Figs. 2–4, respectively. From these micrographs it is evident that the grain size in the parent metal is > Heat-Affected Zone (HAZ) > weld zone in all the conditions of austenitization. Grain size measurements showed that, in general weld zone and HAZ exhibited finer grain size as compared to the parent metal and weld zone grain size was finer than HAZ grain size (Fig. 5). Grain size in all the regions increased with an increase in the austenitization temperature. However, the grain size trends in
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Fig. 8. Optical microstructures of parent metal in different heat treated conditions (austenitized and tempered) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
the three regions remained constant. SEM examination revealed precipitation of carbides in the weld zone up to an austenitization temperature of 1050 °C and, above 1050 °C parent metal and weld were free from carbides (Fig. 6). The observed uninhibited grain growth after 1050 °C could be due to the absence of carbides that pin the grain boundaries. Retained austenite content increased rapidly with increasing austenitization temperature from 950 °C to 1100 °C beyond which the rate of increase was lower (Fig. 7). Progressive increase in retained austenite content of the steel with increasing austenitization temperature is in agreement with earlier studies [9]. The presence of retained austenite in steels decreases the strength [3,7] and causes distortion as it transforms during tempering
[18]. In order to minimize the formation of retained austenite in the as-quenched steel, the selected austenitization temperature should be as low as possible. The experimental results clearly indicate that with increase in austenitization temperature, the volume fraction of retained austenite gradually increases, the volume fraction of undissolved carbides progressively decreases and the grain size increases from 15 lm to 280 lm for parent metal and 5 lm to 75 lm for weld zone. The finer grain size in the weld region could be due to the cast and unworked condition of this region. Increased austenite content with increase in the austenitization temperature is due to higher carbon content in solution, as carbon is an austenite stabilizer.
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Fig. 9. Optical microstructures of welds in different heat treated conditions (austenitized and tempered) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
3.1.2. Tempered condition The microstructure of the parent metal and welds subjected to austenitization at different temperatures followed by tempering are shown in Figs. 8 and 9. From the figures it is evident that the martensitic microstructure experiences coarsening. The degree of coarsening increases with an increase in the austenitization temperature. The extent of coarsening of martensitic microstructure is observed high in parent metal compared to weld zone. Retained austenite content after tempering was observed to be below 2% irrespective of austenitizing temperature. Reduction in retained austenite content after tempering is due to the decomposition of austenite to M23C6 carbides and ferrite by the reaction c ? M23C6 + a.
3.2. Hardness 3.2.1. As-quenched condition The influence of austenitization temperature on as-quenched hardness of parent metal and weld zone is shown in Fig. 10. The hardness is observed to increase from 467 Hk for parent metal and 474 Hk for weld zone at the austenitizing temperature of 950 °C to a peak value of 576 Hk for parent metal and 587 Hk for the weld zone at 1100 °C, beyond which the hardness decreased. Hardness of weld zone is higher compared to corresponding parent metal irrespective of austenitizing treatment possibly due to presence of fine prior austenite grain structure in the weld zone. A significant hardness peak is observed in the as-quenched condition
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when austenitized in the temperature range 1050–1100 °C. This is in agreement with the results of Brownrigg [2]. The high hardness at higher austenitizing temperature is thought to be due to higher degree of quenching coupled with reduction in Ms as a result of carbon in solution. Reduced Ms results in finer martensitic lath microstructure. The lower hardness of the steel when austenitized below 1000 °C is due to large volume fraction of undissolved carbides in the parent metal and precipitated carbides in the weld zone resulting in martensite that is lean in carbon. It is well-known that the strength of martensite increases with increasing carbon content. More the population of undissolved carbides lower is the carbon in austenite and softer is the martensite. Austenitizing at 1050 °C and above allows maximum carbide to go into solution forming a carbon rich martensite on quenching with corresponding increase in hardness. However, austenitizing beyond 1100 °C causes a slight decrease in hardness, that can be attributed to excessive grain growth, increased amount of d-ferrite and retained austenite in the steel without significantly affecting the carbon content of the martensite [9]. 3.2.2. Tempered condition When the welds are subjected to tempering, hardness reduction occurs (Fig. 10). However, the hardness trends are similar to those observed in the as-welded condition with the exception of samples subjected to 1150 °C austenitization in which condition the hardness is increased marginally. Samples austenitized between 1050 °C and 1150 °C resulted in marginal improvement in the hardness compared to 950 °C in the hardened and tempered condition. However, the difference in the tempered hardness of samples
Fig. 11. Appearance of EB welded tensile test specimen after failed out side the weld (arrow indicates weld joint).
Table 4 Impact properties of parent metal and EB welds in different heat treated conditions Material condition
Impact toughness, J/cm2 Parent metal Weld
Austenitized temperature, °Ca 950
1000
1050
1100
1150
80 72
90 76
115 98
110 102
92 80
a Austenitization followed by double tempering, i.e., tempered at 670 °C for 2 h followed by air cooling and subsequently tempering at 600 °C for 2 h followed by air cooling.
austenitized between 1050 °C and 1150 °C is not significant. The marginal improvement in the hardness may be explained as follows: The hardness of austenitized and tempered samples depends on grain size, dissolution of carbides during austenitizing treatment and carbide precipitation after tempering. Lower
Fig. 10. Hardness of parent metal and welds in different heat treated conditions (austenitized and tempered).
Table 3 Tensile properties of EB welds in different heat treated conditions Material condition
0.2% Yield strength, MPa
Ultimate tensile strength, MPa
Elongation, %
Failure location
950 °C austenitization + double tempering 1000 °C austenitization + double tempering 1050 °C austenitization + double tempering 1100 °C austenitization + double tempering 1150 °C austenitization + double tempering
664 750 770 825 682
929 980 1000 1080 835
6.8 10 12 13.5 8.2
Out Out Out Out Out
side side side side side
the the the the the
weld weld weld weld weld
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Fig. 12. Fracture features of impact samples of parent metal in different heat treated conditions (austenitized and tempered) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
austenitizing temperatures resulted low dissolution of carbides with fine grain size [2]. Finer grain size could be responsible for an increased rate of precipitation owing to a larger grain boundary area available for nucleation and the martensitic matrix will be softer. With increase in austenitizing temperature, the grain size increased to maximum at 1150 °C, which resulted low precipitation of carbides and hence the marginal increase in hardness is observed in samples austenitized at 1150 °C. This is in agreement with the results of Brownrigg [2]. 3.3. Tensile properties The variations in 0.2% yield strength (YS), ultimate tensile strength (UTS) percentage elongation (%El) with variation in
austenitizing temperature (950–1150 °C) and tempered at 670 °C + 600 °C are presented in Table 3. The failure location of the weld joint is in the parent metal irrespective of the post-weld austenitizing and tempering treatment (Fig. 11). Failure in the parent metal indicates that weld regions are stronger than the parent metal. YS and UTS of weldments reached a peak value of 825 MPa and 1080 MPa, respectively with increase in austenitization temperature to 1100 °C. Austenitization beyond 1100 °C led to a drastic reduction in both YS and UTS reaching a low of 682 MPa and 835 MPa, respectively. The observed increase in strength with an increase in the austenitizing temperature up to 1100 °C could be due to strengthening of the matrix with carbon in solution and these results are consistent with earlier reports of martensitic stainless steels of type AISI 422 and 403 [19–21]. Excessive grain
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Fig. 13. Fracture features of impact samples of welds in different heat treated conditions (austenitized and tempered) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
growth resulted in decrease in strength at an austenitizing temperature of 1150 °C. Since the failure of the tensile samples is outside the weld, the properties correspond to parent metal. The observed influence of increased austenitization temperature on enhancing the strength could be due to the fine martensite microstructure as a result quenching from higher temperatures. Decrease in the strength after 1150 °C austenitization is thought to be due to the presence of retained d-ferrite. 3.4. Impact toughness Impact toughness data of parent metal and welds normalized as per standard specimen are presented in Table 4. Welds exhibited
lower toughness as compared to parent metal irrespective of the post-weld austenitizing and tempering treatments. Toughness increased with an increase in austenitizing temperature up to 1100 °C and thereafter toughness reduced. The toughness improvement after tempering could be attributed to coarser intragranular microstructure that would blunt an advancing crack necessitating higher energy for further crack extension. Lower toughness of parent metal and weld zone above 1100 °C is in conformity with mixed mode of cleavage and dimpled rupture seen in impact specimens. The higher levels of toughness (1050 °C, 1100 °C) correlated well with equiaxed dimple fracture of the specimens (Figs. 12 and 13). Improvement in toughness was reflected in crack path. With improvement in toughness the crack path
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Fig. 14. Macroscopic views of impact tested samples along with crack path features. (a), (b) and (c) Quenched at 950 °C and tempered; (d), (e) and (f) quenched at 1100 °C and tempered.
Fig. 15. Influence of austenitizing temperature on impact toughness and crack path length.
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was more tortuous (Fig. 14). Crack path length measurement showed toughness improvement is due to increase in crack path length (Fig. 15). These observations are in conformity with similar trends reported in steels and titanium alloys [22–24]. Lower toughness of welds as compared to the parent metal could be due to presence of less number of martensite colonies that contribute to crack path deviation.
their continued encouragement and permission to publish this work. We are thankful to Mr. C.V.S. Murty, DRDL for allowing us in utilizing the electron beam welding facility. One of the authors (A. Rajasekhar) is thankful to the Principal and the Management of Sri Visvesvaraya Institute of Technology and Science (SVITS) for their continued support during this work. Financial assistance from Defence Research Development Organization (DRDO) is gratefully acknowledged.
4. Conclusions The effect of austenitizing temperature on microstructure and mechanical properties of electron beam welds of AISI 431 martensitic stainless steel has been studied. The following were the salient conclusions: (i) Weld zone consisted of fine grain size as compared to the parent metal. Retained austenite content increased with an increase in the austenitizing temperature. (ii) The hardness of welds increased with increase in the austenitizing temperature up to 1100 °C and a marginal decrease thereafter was observed. The increased hardness is due to dissolution of carbides with increase in the austenitizing temperature resulting in matrix strengthening with carbon in solution aided by higher cooling rates when quenched from higher temperatures. Marginal reduction in hardness after 1150 °C could be the result of a combination of factors consisting of excessive grain growth, retained delta ferrite and austenite. (iii) Double tempering led to decrease in hardness as a result of carbide precipitation. However, the trend of higher hardness with increasing austenitizing temperature persisted. (iv) Strength and toughness increased with an increase in the austenitizing temperature up to 1100 °C. Above 1100 °C strength decrease was observed due to excessive grain growth. Failure of the tensile samples occurred outside the weld zone suggesting that welds are stronger than the parent metal that can be attributed to the finer grain size in the weld regions. (v) Toughness improvement was observed due to tortuous crack path leading to increased crack path length. Acknowledgements The authors would like to thank Dr. G. Malakondaiah, Director, Defence Metallurgical and Research Laboratory, Hyderabad for
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Influence of austenitizing temperature on microstructure and mechanical properties of AISI 431 martensitic stainless steel electron beam welds A. Rajasekhar a,*, G. Madhusudhan Reddy b, T. Mohandas b, V.S.R. Murti a a b
Department of Mechanical Engineering, SVITS, Mahaboob Nagar 509 001, India Defence Metallurgical Research Laboratory, Hyderabad 500 058, India
a r t i c l e
i n f o
Article history: Received 25 May 2008 Accepted 18 July 2008 Available online 31 July 2008 Keywords: Martensitic stainless steel Electron beam welding Austenitization temperature Microstructure Mechanical properties
a b s t r a c t The relative effects of various austenitizing temperatures on microstructure and mechanical properties of electron beam welds of AISI 431martensitic stainless steel were studied. The post-weld heat treatments consist of austenitizing the weld samples for 1 h at various temperatures, i.e., at 950 °C, 1000 °C, 1050 °C, 1100 °C and at 1150 °C and air cooling followed by double tempering, i.e., tempering at 670 + 600 °C. In the as-welded condition the microstructure contains dendritic structure with ferrite network and retained austenite in a matrix of un-tempered martensite. The prior austenite grain size increased with increase in austenitizing temperature. Parent metal grain size was coarser as compared to grain size in the weld zone in respective conditions. Retained austenite content increased with increase in the austenitizing temperature. Presence of undissolved carbides was observed in welds and parent metal austenitized up to 1000 °C and they dissolved at austenitizing temperature P 1050 °C. Coarsening of martensite laths was observed after tempering. The martensite laths were coarser in the samples subjected to higher austenitizing temperatures. Optimum mechanical properties, i.e., strength, hardness and toughness were observed when austenitized between 1050 °C and 1100 °C followed by tempering. Austenitizing at 1150 °C and tempering resulted in inferior mechanical properties. The mechanism for the observed trends is discussed in relation to the microstructure, fracture features and mechanical properties. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Martensitic stainless steels containing 17Cr–2Ni are commonly used in quenched and tempered condition. The heat treatment consists of annealing to obtain austenite and dissolve the carbides, followed by cooling to transform the austenite into martensite and subsequent tempering of martensitic microstructure. Depending on the composition and processing history, the microstructure of martensitic stainless steel consists of martensite, undissolved carbides as well as reprecipitated carbides, retained austenite and dferrite. It is well-known that properties obtained in these steels are strongly influenced by such treatments. The amount of carbides in the as-quenched microstructure exerts an influence on the properties of these materials such as hardness, strength, toughness, corrosion and wear [1]. Earlier studies on 17Cr–2Ni martensitic stainless steel have revealed that due to the composition imbalance between austenite and ferrite stabilizing elements in
* Corresponding author. Tel.: +91 9989 421206; fax: +91 8542 236920. E-mail address: [email protected] (A. Rajasekhar). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.042
the steel and inappropriate austenitizing temperatures employed, the steel could end up with microstructure containing large amount of retained austenite, d-ferrite and carbides in a martensitic matrix [2,3]. The criteria for a suitable alloy design demands a low content of d-ferrite as it is deleterious for strength [4,5], toughness [6,7] and corrosion resistance [2,8] and maximum amount of carbon in solution for greater hardness and strength of martensite [9]. Some of the previous studies revealed that volume fraction of d-ferrite does not change up to an austenitizing temperature of 1100 °C, but increases with increasing austenitizing temperature above this temperature [2,9]. This observation indicates that up to <1100 °C, the 17Cr–2Ni steel remained in the c-phase field and entered into (c + d) phase field above 1100 °C. Therefore, below 1100 °C for a given composition of the steel, the strength mainly depends on the extent of dissolution of carbides and alloying elements in the austenite, which on quenching forms strong martensite. The hardness and strength of as-quenched martensite will dictate the strength levels achievable on tempering at suitable temperatures. The recommended austenitizing temperatures by various international standards based on composition and heat treatments lies anywhere between 950 °C and 1100 °C. However, these recommendations do not appear to be supported
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Table 1 Chemical composition of martensitic stainless steel type AISI 431 Element
C
Si
Mn
Cr
Ni
S
P
Weight (%)
0.17
0.6
1.0
17.0
2.0
0.03
0.04
Table 2 Welding parameters for EB welding Machine settings/parameters Gun to work distance, mm Accelerating voltage, KV Beam current, mA Focus Speed, m/min Vacuum level, mbar Heat input, J/mm
283 55 35 mA (for initial first pass for pre heating) and 65 mA (for penetration) Slightly above the surface 1.0 10 4 mbar and less 214.5
Fig. 1. Location and orientation of the Charpy V-notch impact sample in the weld zone (schematic).
by systematic experimental investigations correlating properties to microstructure. Considerable research work has been carried out
Fig. 2. Optical microstructures of parent metal in different austenitized conditions (before tempering) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
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Fig. 3. Optical microstructures of EB weld interfaces in different austenitized conditions (before tempering) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
on 17Cr–2Ni martensitic stainless steel by Balan et al. [10] in the direction of correlating microstructure with properties by varying austenitizing temperatures. The optimum austenitization temperature of the 17Cr–2Ni steel is found to lie between 1050 °C and 1100 °C w.r.t. to dissolution of carbides, minimum d-ferrite and fine grain size that resulted in the best combination of hardness, strength, ductility and notch toughness in the as-quenched condition. Since martensitic stainless steels are structural materials, weldability has been an important consideration in their development. In general, these steels are readily weldable by conventional welding processes, although special precautions are necessary to avoid
hydrogen cracking because of the low Ms temperature and the propensity of forming fine and brittle martensite even at normal air cooling rates [11]. Welding procedures are designed primarily to avoid hydrogen cracking and to obtain adequate weldment toughness [12]. To weld martensitic stainless steels both preheat and post-weld heat treatments (PWHT) are necessary to reduce the hardness of the martensitic regions thereby ensuring the structural integrity of the weld region during service. Conventional arc welding characteristics of martensitic stainless steels have been examined by several researchers [13–16]. However, data on high energy density processes such as electron beam (EB) welding are scarce. Electron beam welding is character-
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Fig. 4. Optical microstructure of EB welds in different austenitized conditions (before tempering) (a) 950 °C (b) 1000 °C (c) 1050 °C (d) 1100 °C and (e) 1150 °C.
ized by a short interaction time with intense energy density. As a result, the process has a lower heat input, which is expected to exercise greater control over grain coarsening and distortion. EB welding can offer advantage on higher joint completion rates for a variety of engineering fabrications. The influence of post-weld heat treatments on microstructure and mechanical properties of electron beam welds of AISI 431 martensitic stainless steel has been evaluated by Rajasekhar et al. [17], by employing different post-weld heat treatments, consisting of austenitizing the weld samples at 950 °C and air cooling followed by tempering at 400 °C, 600 °C, 670 °C and 670 + 600 °C. It has been reported that although 950 °C is about 50–100 °C above the upper critical temperature
(Ac3) for this steel, depending on the carbon and chromium contents, the steel might require a higher austenitization temperature for complete dissolution of chromium carbides [2,9]. However, grain coarsening, retention of austenite, and formation of d-ferrite in the as-quenched microstructure need to be taken in to consideration while selecting a suitable austenitizing temperature. Published information available regarding welding behavior of martensitic stainless steel AISI 431 is very limited. The lack of knowledge constitutes a potential drawback to the more widespread use of welded martensitic stainless steels. Keeping the above in view, it is envisaged to evaluate the influence of austenitization temperature on the microstructure
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Fig. 5. Grain size of parent metal, HAZ and welds in different austenitized conditions.
Fig. 6. SEM microstructures of parent metal and welds in as-quenched condition. (a) Parent metal at 950 °C; (b) parent metal at 1050 °C; (c) weld at 950 °C and (d) weld at 1050 °C.
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Fig. 7. Influence of austenitizing temperature on retained austenite.
and mechanical properties of 17Cr–2Ni steel welds. Parent metal was also subjected to similar treatments for comparative evaluation.
2. Experimental details The parent material used in this study is AISI 431 martensitic stainless steel whose chemical composition is given in Table 1. The material was received in 30 mm diameter rod form in fully annealed condition. It is then forged at 1200 °C to 5 mm thick plates. Forged plates were annealed at 670 °C for 1 h and air cooled prior to welding. Autogenous bead on plate full penetration electron beam welding of the plates was carried out. The welding parameters are given in Table 2. Initial low power welding has been adopted to preheat the material. Welded samples were austenitized for 1 h at selected temperatures in the range 950–1150 °C with an interval of 50 °C followed by air cooling to room temperature. The air cooled samples were subsequently double tempered, i.e., tempered at 670 °C for 2 h followed by air cooling and subsequently tempering at 600 °C for 2 h followed by air cooling. For comparison purpose parent metal has also been subjected to similar heat treatments. Mechanical properties, i.e., micro-hardness, transverse tensile strength and Charpy ‘V’-notch impact toughness of welds and parent metal in different heat treated conditions were evaluated. Tensile and Charpy V-notch specimens were made to sub-size as per ASTM standards, and were tested at room temperature. Notch is located in the centre of the weld joint. The location and orientation of the Charpy V-notch impact sample in the weld zone is shown in Fig. 1. Micro-hardness survey was carried across the joint interface at the centre on a Matsuzawa micro-hardness tester using a Knoop indenter. Standard metallographic samples were prepared and examined for microstructural details under Leitz optical and stereo microscopes after etching. Kallings reagent (cupric chloride – 1.5 g, HCl – 33 ml, ethanol – 33 ml and distilled water – 33 ml) was used to reveal the microstructure after tempering. Electrolytic etchant
(60 parts nitric acid, HNO3 and 40 parts H2O) was used to reveal the prior austenite grain boundaries. Retained austenite (cr) content in the welds and parent metal in various heat treated conditions was estimated using Stress Tech 3000 X-ray system using Cr Ka radiation. The volume fraction of austenite was estimated from measurements of the integrated intensities of martensite, ferrite and austenite peaks assuming they are the only phases present. Fractography of the tensile and impact samples was carried by employing LEO scanning electron microscope (SEM). The impact samples were sectioned in mid section perpendicular to the notch to examine crack path under an optical microscope. The prior austenite grain size of quenched parent and weld zones in various austenitized conditions were measured using linear intercept methods. At least eight intercepts across the length, breadth and diagonal of the photographs at a magnification of 100 were taken on five photographs chosen from different areas of the sample. 3. Results and discussion 3.1. Microstructure 3.1.1. As-quenched microstructure Optical metallography was carried out in the as-quenched condition to identify the variation in the gross features of microstructure, e.g. prior austenite grain size, retained austenite content and undissolved carbides of the parent metal and welds austenitized at 950 °C, 1000 °C, 1050 °C, 1100 °C and 1150 °C. Typical optical microstructures at the fusion boundary, in the parent metal and weld zone in different austenitized conditions are shown in Figs. 2–4, respectively. From these micrographs it is evident that the grain size in the parent metal is > Heat-Affected Zone (HAZ) > weld zone in all the conditions of austenitization. Grain size measurements showed that, in general weld zone and HAZ exhibited finer grain size as compared to the parent metal and weld zone grain size was finer than HAZ grain size (Fig. 5). Grain size in all the regions increased with an increase in the austenitization temperature. However, the grain size trends in
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Fig. 8. Optical microstructures of parent metal in different heat treated conditions (austenitized and tempered) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
the three regions remained constant. SEM examination revealed precipitation of carbides in the weld zone up to an austenitization temperature of 1050 °C and, above 1050 °C parent metal and weld were free from carbides (Fig. 6). The observed uninhibited grain growth after 1050 °C could be due to the absence of carbides that pin the grain boundaries. Retained austenite content increased rapidly with increasing austenitization temperature from 950 °C to 1100 °C beyond which the rate of increase was lower (Fig. 7). Progressive increase in retained austenite content of the steel with increasing austenitization temperature is in agreement with earlier studies [9]. The presence of retained austenite in steels decreases the strength [3,7] and causes distortion as it transforms during tempering
[18]. In order to minimize the formation of retained austenite in the as-quenched steel, the selected austenitization temperature should be as low as possible. The experimental results clearly indicate that with increase in austenitization temperature, the volume fraction of retained austenite gradually increases, the volume fraction of undissolved carbides progressively decreases and the grain size increases from 15 lm to 280 lm for parent metal and 5 lm to 75 lm for weld zone. The finer grain size in the weld region could be due to the cast and unworked condition of this region. Increased austenite content with increase in the austenitization temperature is due to higher carbon content in solution, as carbon is an austenite stabilizer.
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Fig. 9. Optical microstructures of welds in different heat treated conditions (austenitized and tempered) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
3.1.2. Tempered condition The microstructure of the parent metal and welds subjected to austenitization at different temperatures followed by tempering are shown in Figs. 8 and 9. From the figures it is evident that the martensitic microstructure experiences coarsening. The degree of coarsening increases with an increase in the austenitization temperature. The extent of coarsening of martensitic microstructure is observed high in parent metal compared to weld zone. Retained austenite content after tempering was observed to be below 2% irrespective of austenitizing temperature. Reduction in retained austenite content after tempering is due to the decomposition of austenite to M23C6 carbides and ferrite by the reaction c ? M23C6 + a.
3.2. Hardness 3.2.1. As-quenched condition The influence of austenitization temperature on as-quenched hardness of parent metal and weld zone is shown in Fig. 10. The hardness is observed to increase from 467 Hk for parent metal and 474 Hk for weld zone at the austenitizing temperature of 950 °C to a peak value of 576 Hk for parent metal and 587 Hk for the weld zone at 1100 °C, beyond which the hardness decreased. Hardness of weld zone is higher compared to corresponding parent metal irrespective of austenitizing treatment possibly due to presence of fine prior austenite grain structure in the weld zone. A significant hardness peak is observed in the as-quenched condition
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when austenitized in the temperature range 1050–1100 °C. This is in agreement with the results of Brownrigg [2]. The high hardness at higher austenitizing temperature is thought to be due to higher degree of quenching coupled with reduction in Ms as a result of carbon in solution. Reduced Ms results in finer martensitic lath microstructure. The lower hardness of the steel when austenitized below 1000 °C is due to large volume fraction of undissolved carbides in the parent metal and precipitated carbides in the weld zone resulting in martensite that is lean in carbon. It is well-known that the strength of martensite increases with increasing carbon content. More the population of undissolved carbides lower is the carbon in austenite and softer is the martensite. Austenitizing at 1050 °C and above allows maximum carbide to go into solution forming a carbon rich martensite on quenching with corresponding increase in hardness. However, austenitizing beyond 1100 °C causes a slight decrease in hardness, that can be attributed to excessive grain growth, increased amount of d-ferrite and retained austenite in the steel without significantly affecting the carbon content of the martensite [9]. 3.2.2. Tempered condition When the welds are subjected to tempering, hardness reduction occurs (Fig. 10). However, the hardness trends are similar to those observed in the as-welded condition with the exception of samples subjected to 1150 °C austenitization in which condition the hardness is increased marginally. Samples austenitized between 1050 °C and 1150 °C resulted in marginal improvement in the hardness compared to 950 °C in the hardened and tempered condition. However, the difference in the tempered hardness of samples
Fig. 11. Appearance of EB welded tensile test specimen after failed out side the weld (arrow indicates weld joint).
Table 4 Impact properties of parent metal and EB welds in different heat treated conditions Material condition
Impact toughness, J/cm2 Parent metal Weld
Austenitized temperature, °Ca 950
1000
1050
1100
1150
80 72
90 76
115 98
110 102
92 80
a Austenitization followed by double tempering, i.e., tempered at 670 °C for 2 h followed by air cooling and subsequently tempering at 600 °C for 2 h followed by air cooling.
austenitized between 1050 °C and 1150 °C is not significant. The marginal improvement in the hardness may be explained as follows: The hardness of austenitized and tempered samples depends on grain size, dissolution of carbides during austenitizing treatment and carbide precipitation after tempering. Lower
Fig. 10. Hardness of parent metal and welds in different heat treated conditions (austenitized and tempered).
Table 3 Tensile properties of EB welds in different heat treated conditions Material condition
0.2% Yield strength, MPa
Ultimate tensile strength, MPa
Elongation, %
Failure location
950 °C austenitization + double tempering 1000 °C austenitization + double tempering 1050 °C austenitization + double tempering 1100 °C austenitization + double tempering 1150 °C austenitization + double tempering
664 750 770 825 682
929 980 1000 1080 835
6.8 10 12 13.5 8.2
Out Out Out Out Out
side side side side side
the the the the the
weld weld weld weld weld
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Fig. 12. Fracture features of impact samples of parent metal in different heat treated conditions (austenitized and tempered) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
austenitizing temperatures resulted low dissolution of carbides with fine grain size [2]. Finer grain size could be responsible for an increased rate of precipitation owing to a larger grain boundary area available for nucleation and the martensitic matrix will be softer. With increase in austenitizing temperature, the grain size increased to maximum at 1150 °C, which resulted low precipitation of carbides and hence the marginal increase in hardness is observed in samples austenitized at 1150 °C. This is in agreement with the results of Brownrigg [2]. 3.3. Tensile properties The variations in 0.2% yield strength (YS), ultimate tensile strength (UTS) percentage elongation (%El) with variation in
austenitizing temperature (950–1150 °C) and tempered at 670 °C + 600 °C are presented in Table 3. The failure location of the weld joint is in the parent metal irrespective of the post-weld austenitizing and tempering treatment (Fig. 11). Failure in the parent metal indicates that weld regions are stronger than the parent metal. YS and UTS of weldments reached a peak value of 825 MPa and 1080 MPa, respectively with increase in austenitization temperature to 1100 °C. Austenitization beyond 1100 °C led to a drastic reduction in both YS and UTS reaching a low of 682 MPa and 835 MPa, respectively. The observed increase in strength with an increase in the austenitizing temperature up to 1100 °C could be due to strengthening of the matrix with carbon in solution and these results are consistent with earlier reports of martensitic stainless steels of type AISI 422 and 403 [19–21]. Excessive grain
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Fig. 13. Fracture features of impact samples of welds in different heat treated conditions (austenitized and tempered) (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C and (e) 1150 °C.
growth resulted in decrease in strength at an austenitizing temperature of 1150 °C. Since the failure of the tensile samples is outside the weld, the properties correspond to parent metal. The observed influence of increased austenitization temperature on enhancing the strength could be due to the fine martensite microstructure as a result quenching from higher temperatures. Decrease in the strength after 1150 °C austenitization is thought to be due to the presence of retained d-ferrite. 3.4. Impact toughness Impact toughness data of parent metal and welds normalized as per standard specimen are presented in Table 4. Welds exhibited
lower toughness as compared to parent metal irrespective of the post-weld austenitizing and tempering treatments. Toughness increased with an increase in austenitizing temperature up to 1100 °C and thereafter toughness reduced. The toughness improvement after tempering could be attributed to coarser intragranular microstructure that would blunt an advancing crack necessitating higher energy for further crack extension. Lower toughness of parent metal and weld zone above 1100 °C is in conformity with mixed mode of cleavage and dimpled rupture seen in impact specimens. The higher levels of toughness (1050 °C, 1100 °C) correlated well with equiaxed dimple fracture of the specimens (Figs. 12 and 13). Improvement in toughness was reflected in crack path. With improvement in toughness the crack path
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Fig. 14. Macroscopic views of impact tested samples along with crack path features. (a), (b) and (c) Quenched at 950 °C and tempered; (d), (e) and (f) quenched at 1100 °C and tempered.
Fig. 15. Influence of austenitizing temperature on impact toughness and crack path length.
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was more tortuous (Fig. 14). Crack path length measurement showed toughness improvement is due to increase in crack path length (Fig. 15). These observations are in conformity with similar trends reported in steels and titanium alloys [22–24]. Lower toughness of welds as compared to the parent metal could be due to presence of less number of martensite colonies that contribute to crack path deviation.
their continued encouragement and permission to publish this work. We are thankful to Mr. C.V.S. Murty, DRDL for allowing us in utilizing the electron beam welding facility. One of the authors (A. Rajasekhar) is thankful to the Principal and the Management of Sri Visvesvaraya Institute of Technology and Science (SVITS) for their continued support during this work. Financial assistance from Defence Research Development Organization (DRDO) is gratefully acknowledged.
4. Conclusions The effect of austenitizing temperature on microstructure and mechanical properties of electron beam welds of AISI 431 martensitic stainless steel has been studied. The following were the salient conclusions: (i) Weld zone consisted of fine grain size as compared to the parent metal. Retained austenite content increased with an increase in the austenitizing temperature. (ii) The hardness of welds increased with increase in the austenitizing temperature up to 1100 °C and a marginal decrease thereafter was observed. The increased hardness is due to dissolution of carbides with increase in the austenitizing temperature resulting in matrix strengthening with carbon in solution aided by higher cooling rates when quenched from higher temperatures. Marginal reduction in hardness after 1150 °C could be the result of a combination of factors consisting of excessive grain growth, retained delta ferrite and austenite. (iii) Double tempering led to decrease in hardness as a result of carbide precipitation. However, the trend of higher hardness with increasing austenitizing temperature persisted. (iv) Strength and toughness increased with an increase in the austenitizing temperature up to 1100 °C. Above 1100 °C strength decrease was observed due to excessive grain growth. Failure of the tensile samples occurred outside the weld zone suggesting that welds are stronger than the parent metal that can be attributed to the finer grain size in the weld regions. (v) Toughness improvement was observed due to tortuous crack path leading to increased crack path length. Acknowledgements The authors would like to thank Dr. G. Malakondaiah, Director, Defence Metallurgical and Research Laboratory, Hyderabad for
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