Elsevier

Materials Letters

Volume 264, 1 April 2020, 127321
Materials Letters

Grain boundary engineering to overcome temper embrittlement in martensitic steel

https://doi.org/10.1016/j.matlet.2020.127321Get rights and content

Highlights

  • Q&P heat treatment stabilizes retained austenite.

  • Volume fraction of austenite increases with increased quench temperature.

  • Presence of austenite modify GBCD significantly.

  • Increased CSL fraction reduces interfacial energy.

  • Presence of austenite overcome temper embrittlement during tempering in 12Cr steel.

Abstract

Grain boundary engineering (GBE) approach has been introduced in martensitic steel through quenching and partitioning (Q&P) heat treatment to overcome temper embrittlement. This heat treatment increases fraction of low energy coincidence site lattice (CSL) boundary from 0.12 in Q&P specimen to 0.22 in Q&P-tempered specimen through transformation of retained austenite to martensite. Presence of austenite in martensitic matrix produces different interfaces. Low energy austenite / austenite and austenite / martensite interfaces reduces segregation of tramp elements during tempering leading to increased toughness compared to normalized and tempered specimen having high energy martensite / martensite interface. Increased toughness is accompanied by a change in fracture morphology from intergranular fracture to typical dimple fracture.

Introduction

GBE concept has been introduced in various polycrystalline materials and their alloys to improve strength, ductility, stress corrosion resistance, creep and intergranular damage resistance [1], [2], [3], [4], [5]. This technique has been successfully employed in many iron based austenitic stainless steel and nickel base alloys having low stacking fault energy, favoring the formation of twins during deformation followed by annealing or TMT (thermo-mechanical treatment) [6], [7], [8], [9]. GBE is less popular in iron based body centered cubic system due to its high stacking fault energy (SFE). It is difficult to implement GBE concept in martensitic steel due to its complex microstructure having precipitates at different boundaries and within lath [10]. These precipitates pin the boundaries and restrict their migration during tempering. Grain boundary character distributions (GBCD) in 9-12Cr ferritic steels have been used to characterize microstructure [11]. But, till today no effort has been made to overcome temper embrittlement in martensitic steels through GBE approach. In GBE approach, generations of low energy CSL boundary will reduce segregation of tramp elements. In martensitic steel, CSL boundary are related to the martensitic phase transformation. Hence, for a given composition CSL boundary fraction is fixed. If martensitic transformation can be performed in different stages of cooling by modifying composition of austenite, fraction of CSL boundary will increase. This is possible if carbon can be partitioned between martensite and austenite while cooling. Q&P process is established based on partitioning of elements from martensite to austenite [12]. Therefore, the objective of the present investigation is to overcome temper embrittlement in martensitic steel using this concept [13].

Section snippets

Experimental procedure

12Cr martensitic steel plate of 12 mm thick was used and cut into sizes of 12 × 12 × 57 mm3 and 122 × 12 × 12 mm3. One set of specimen were solutionised at 980 °C for 30 min, followed by quenching and partitioned at 150, 175, 200, 225, 250 and 275 °C for 2 h in a salt bath furnace. This set of heat treatment is defined as Q&P. Austenite and martensite phases were stable in these temperature ranges as Ms (Martensite start) and Mf (Martensite finish) temperatures of the steel are 316 and 130 °C

Results and discussion

Toughness value of N and N&T specimens were found to be 5 and 7 J respectively. It increases monotonically from 36 to 76 J for Q&P specimens, with increasing Q&P temperature from 150 to 225 °C, but decreases to 27 J at 250 °C from the peak value. XRD peak analysis reveals the presence of austenite phase in addition to martensite phase only in those specimens which were Q&P at 200, 225 and 250 °C and the volume fractions of austenite is 1.3, 4 and 9.8% respectively. Austenite was absent in

Conclusions

Q&P heat treatment between Ms and Mf temperature stabilizes austenite. Heat treatment at higher temperature increases the volume fraction of austenite but reduces toughness of the steel. Q&P and Q&P-tempered heat treatment significantly modify GBCD in martensitic steel. Presence of lower fraction of retained austenite reduces fraction of Σ3 boundary but increases grain boundary line length per unit area. Tempering of Q&P steel increases fraction of Σ3 boundaries. Low energy boundary reduces

CRediT authorship contribution statement

Kirtiratan Godbole: Investigation. C.R. Das: Conceptualization, Writing - original draft. Bharat B. Panigrahi: Project administration and Writing - review editing. S.K. Albert: Supervision.

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.

Acknowledgements

Authors acknowledge the support and encouragement received from Director, IGCAR for this work. Authors also acknowledge the TEM facility of NIT Surathkal and DST (Govt. of India) for FIST Project for SEM at IIT Hyderabad.

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