The heat affected zone of X20Cr13 martensitic stainless steel after multiple repair welding: Microstructure and mechanical properties assessment

doi:10.1016/j.ijpvp.2020.104205

International Journal of Pressure Vessels and Piping

Volume 188, December 2020, 104205

https://doi.org/10.1016/j.ijpvp.2020.104205 Get rights and content

Highlights

•
Effect of multiple repair welding on mechanical behavior and microstructure of a martensitic stainless steel is studied.
•
Refinement of austenite grains in the heat affected zone occurred as a result of successive heating and cooling cycles.
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Disintegration of grain boundary carbide network occurred in the heat affected zone.
•
Charpy impact test energy of the HAZ increases due to grain refinement and disintegration of carbide network.

Abstract

Multiple repair welding of X20Cr13 martensitic stainless steel was performed using gas metal arc welding. The groove for repair welding was designed such that the heat affected zone (HAZ) of each repair welding sequence remained the HAZ of the previous repair (i.e. cyclic weld removal and repair welding was done). After post weld heat treatment, microstructure and mechanical properties of the HAZ were investigated during 0–9 repairs. Refinement of primary austenite grains in the HAZ was observed (from 2.8 ASTM grain size in the zero-repair sample to 6.3 after the ninth repair), increasing the impact toughness (from 34J to 63J, respectively). Moreover, HAZ micro-hardness decreased as repair welding continued due to sequential disintegration of the chromium carbides network at grain/sub-grain boundaries, promoting ductile fracture behavior. It was finally concluded that multiple repair welding has no detrimental effects on microstructure and mechanical properties of X20Cr13 stainless steel and increases HAZ impact toughness and promotes ductile fracture.

Introduction

Welding is widely utilized in industry on various segments such as pressure vessels, boilers, turbine and compressor components for different reasons including manufacturing, repair, and upgrading purposes. Among these, repair welding plays a crucial economic role by eliminating the necessity of replacing the part. This process may be done on components either in operation, or during manufacturing [[1], [2], [3], [4], [5], [6]]. Multiple repair welding, which means performing repair welding in the same spot for multiple times, becomes a necessity when the first repair welding operation yields unacceptable weld, requiring the component to be repaired again. However, due to introduction of massive heat into the HAZ of the segment, different standards have set limitations on the number of multiple repair welding. Thus, multiple repair welding has recently been studied by researchers [[7], [8], [9], [10]].

Standards like DNV-OS-F101 [11], GB50236-98 [12] and IPS-C-PI-270 [13] offer limitations on the number of repair welds for specific applications. DNV-OS-F101 (Appendix C, sub-section G 300) [11] and GB50236-98 [12] standards have limited the number of repair welds to two times and IPS-C-PI-270 to only one repair welding. On the other hand, references such as ASME Sec IX [14] and API-1104 [15] did not state any limitations. There are also several researches on simulation of residual stress in repair welding [[16], [17], [18], [19], [20], [21]]. However, few experimental results were reported regarding mechanical and microstructural behavior of repaired segments [[22], [23], [24], [25]]. In addition, no study was found on multiple repair welding of martensitic stainless steels.

The effect of four times of multiple repair welding on mechanical and microstructural properties of AISI 316L stainless steel using shield metal arc welding (SMAW) technique was studied [22]; as the repairs sequence progressed, δ-ferrite transformed to austenite in the HAZ and its volume fraction decreased, accordingly. In addition, the morphology of δ-ferrite changed from lathy to vermicular and growth of austenite grains occurred. Moreover, these changes lead to significant decrease in HAZ hardness and Charpy impact toughness. Furthermore, HAZ became more sensitive to pitting and crevice corrosion due to mentioned changes as multiple repair welding continued. Tensile results showed that repeated repair welds did not have significant detrimental effect on yield and ultimate tensile strength. It was finally concluded that no more than 2 times of repeated repair welding suggested for parts with chloride-containing environments applications.

In another investigation, the effect of four times of repeated girth repair welding on the microstructure and mechanical properties of API X52 PSL2 micro-alloyed steel was evaluated [23]. Grain refinement occurred in the first repair sequence, which was followed by grain growth as multiple repair welding continued. As a result, Vickers hardness and Charpy impact toughness were found to increase at the first step and experienced a gradual decrease as the procedure continued. Also, all transverse tensile test samples fractured in the base metal.

C.-M. Lin et al. studied microstructural and mechanical behavior of AISI 304L stainless steel after 5 times of multiple repair welding using tungsten inert gas technique [24]. As multiple repairs continued, the volume fraction of lathy ferrite decreased as a result of diffusion and transformation of lathy to short ferrite. In addition, the accumulated heat input during repair welding provided grains formation and increased the fraction of high angle grain boundaries, changing the fracture behavior of Charpy-impact samples from the brittle intergranular with micro-voids to ductile transgranular mode. Moreover, corrosion properties were deteriorated with the progression of repair welding regime due to the increase in the number of corrosion attack sites.

In another study, the effect of multiple repair welding on mechanical properties and microstructure of Q345 base metal cladded with 06Cr19Ni10 austenitic stainless steel was investigated [25]. Formation of a diffusion layer containing martensite was observed around the weld-base metal interface; this increases the risk of cracking. Presence of a short ferrite phase along the fusion zone was also reported. The volume fraction of short ferrite showed increasing at higher numbers of repair weld. In addition, formation of some voids occurred in the third and fourth repair welds; this suggests the complete removal of the diffusion layer before each repair. A ﬁne grain zone beneath the weld formed due to the welding heat; the zone widened with the repair sequence.

Based on all observations, it was proposed that the clad plate should not be repaired more than 2 times. The effects of heat input and number of layers on the residual stress of stainless steel weld were studied by finite element method [16]. The transverse residual stress was found to decrease, while longitudinal residual stress increased with increasing the heat input. However, both stresses decreased as the number of the weld layer increased. It was finally concluded that repair welding with high heat input and multiple layers decreases the residual stress of the repaired weld.

As reported, multiple repair welding studies are mainly done on austenitic stainless steel and for only a few numbers of repair (less than four), and no research was found on martensitic stainless steels or even for higher numbers of repair welding. X20Cr13 martensitic stainless steel (equivalent to AISI 420) is widely used in the manufacturing gas turbines in MAPNA group. During various stages of production, multiple repair welding of this material is required. According to the literature, by optimizing the process parameters of repair welding [[26], [27], [28]]. However, HAZ remains a susceptible region since multiple repair welding may change its microstructure and mechanical properties by the introduction of a massive heat during repair welding cycle. Thus, the main focus of the present work is determination of microstructure and mechanical properties of the HAZ of X20Cr13 martensitic stainless steel after multiple repair welding.

Section snippets

Material and methods

X20Cr13 martensitic stainless steel as base metal and ER 410NiMo as filler material (Table 1) were used to perform multiple repair welding by GMAW process. Work pieces with 500 × 200 × 60 mm dimensions were received in tempered condition. The repair welding groove was designed according to ASME BPV Code Sec III, NB4622.9 (Fig. 1a) and prepared by CNC Boring PAMA Speedmat 4/TR 25 machine such that the HAZ of every repair welding sequence remains the HAZ of the previous one. To ensure that, two

Base metal microstructure

Fig. 2 shows the microstructure of X20Cr13 base metal used in this study. Since the material was hot formed at 800–1100 °C (according to EN 10088-3 standard), the austenite was transformed to martensite. After heat treatment at 650 °C, the microstructure consists of tempered lath martensite (Fig. 2a and b) and residual carbides which have not been dissolved in the matrix during austenitization (Fig. 2c).

HAZ microstructure

Fig. 3a shows HAZ microstructure in 0R specimen. It consists of five sub-zones parallel to

Discussion

Martensitic structure is formed of a prior austenite grain that divided into packets. Packets are filled with extended parallel blocks; these are subdivided by the laths of martensite (Fig. 13a). Lath boundaries are low angle boundaries, while primary austenite grain boundaries, packet boundaries and block boundaries are high angle boundaries [38,39].

After the first cycle of welding (i.e. 0R workpiece), untempered martensite formed in the HAZ adjacent to the fusion line; this introduced a large

Conclusions

In this study, the effect of multiple repair welding on microstructural and mechanical properties of the HAZ of X20Cr13 martensitic stainless steel was studied and the following conclusions resulted:

1.
Successive heating and cooling cycles associated with multiple repair welding refined the austenite grains of the HAZ from 2.8 (first repair) to 6.3 ASTM grain size number (ninth repair), increasing the HAZ impact toughness from 34J to 63J.
2.
Formation of a continuous network of chromium carbides (Cr₂₃C

Author contributions

Mohammad Shojaati is a PhD candidate in the School of Metallurgy and Materials Engineering, University of Tehran under the supervision of Prof. S. F. Kashani-Bozorg.

Dr. Seyed Farshid Kashani-Bozorg is the professor and thesis supervisor in the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran. He is also the director of Center of Excellence for Surface Engineering and Corrosion Protection of Industries.

Masoud Vatanara and Morteza Yazdizadeh are

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported financially by MAPNA Turbine Engineering and Manufacturing Co. under Contract No. 4600000553 (agenda8). The advice and technical support for welding experiments at the Welding Workshop of MAPNA Turbine Engineering and Manufacturing Co. were greatly appreciated. Thanks are also goes to University of Tehran for providing characterization facilities.

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The heat affected zone of X20Cr13 martensitic stainless steel after multiple repair welding: Microstructure and mechanical properties assessment

Highlights

Abstract

Introduction

Section snippets

Material and methods

Base metal microstructure

HAZ microstructure

Discussion

Conclusions

Author contributions

Declaration of competing interest

Acknowledgment

J. Manuf. Process.

Int. J. Pres. Ves. Pip.

Mater. Des.

Int. J. Pres. Ves. Pip.

Int. J. Pres. Ves. Pip.

Int. J. Pres. Ves. Pip.

Mater. Des.

Mater. Char.

Eng. Fail. Anal.

Mater. Des.

Construct. Build. Mater.

Int. J. Plast.

J. Alloys Compd.

Mater. Char.

Mater. Sci. Eng.

Acta Mater.

Scripta Mater.

J. Alloys Compd.

Acta Mater.

Acta Mater.

Acta Mater.

Mater. Sci. Eng.