Microstructure and properties of 13Cr5Ni1Mo0.025Nb0.09V0.06N super martensitic stainless steel

doi:10.1016/j.msea.2012.01.093

Materials Science and Engineering: A

Volume 539, 30 March 2012, Pages 271-279

https://doi.org/10.1016/j.msea.2012.01.093 Get rights and content

Abstract

The morphological microstructure, the density and dispersion of high angle boundaries, morphology and micro chemical composition of precipitates and the volume fraction of retained austenite of a commercial super martensitic stainless steel (SMSS) normalized and tempered at various temperatures were characterized by optical microscope, scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) and X-ray diffraction (XRD) in the light of equilibrium phase diagram of the alloy calculated using Thermo-Calc software. The mechanical properties and pitting corrosion resistance were determined to correlate with microstructures. Two kinds of morphology of precipitate were observed in tempered commercial super martensitic stainless. Besides the globular Nb and V rich carbo-nitride precipitates, rod-like Cr rich nitrides were formed due to excess N content. While high density of high angle boundaries and precipitates contribute to strength properties, the dislocation softening of the matrix and retained austenite from tempering restore the ductility and impact toughness properties. The poor resistance to pitting corrosion is attributed to the occurrence of Cr rich precipitates. It is demonstrated that by lowering the nitrogen content and adding niobium, the Cr rich precipitates can be suppressed and the mechanical properties and resistance to pitting corrosion can be significantly improved.

Highlights

► Characterization of the microstructures of a commercial martenistic stainless steel. ► Excess N content causes the occurrence of coarse carbo-nitride and Cr₂N. ► Correlation of microstructures with mechanical and corrosion properties. ► The poor pitting resistance is due to Cr rich precipitates.

Introduction

Low carbon martensitic stainless steel called super martensitic stainless steel (SMSS) [1], [2], [3], [4] exhibits a unique combination of weldability, strength, toughness and corrosion resistance properties. It has been increasingly applied to critical structures and components such as turbines, marine propellers, aircraft parts and oil country tubular goods – seamless pipes for drilling, casing and tubing – for the application in oil and gas fields with more corrosive environments due to its excellent combined properties, which are actually dependent on chemical compositions and special microstructures of SMSS. The chemical compositions of SMSS are based on Fe–Cr–Ni–Mo system with 4–6% of Ni, 0.5–2.5% of Mo, low C contents (≤0.02%) and varying microalloying and nitrogen content [5], [6], [7]. Martensitic lath microstructure can be obtained even by air cooling SMSSs from solution treatment temperature at austenite field due to their adequate hardenability. Subsequent tempering of SMSSs above A_c1 temperature results in reverse transformation of martensite to austenite, which is relatively stable and can be partially retained at room temperature. The retained austenite has been reported to be very effective to confer the ductility and toughness of SMSSs [8], though at cost of strength. Since Cr and Mo are ferrite stabilizing elements, in martensitic stainless steels, it is necessary to make austenite forming elements additions to avoid the occurrence of delta ferrite [9] so that austenite phase is stabilized and martensitic transformation can occur upon air cooling. Ni is the common alloying element used to stabilize the austenite or expand the gamma phase field, but more Ni addition will add on cost. N is also a very effective austenite stabilizer. Recently, in austenitic and duplex stainless steel, N is employed to substitute Ni, reducing the cost and offering customer excellent combined mechanical properties and corrosion resistance.

By comparison, the N alloying of martensitic stainless steel has been reported rarely as solubility of N in martensite is low. Although N can be used to avoid δ ferrite formation and is also expected to improve the pitting corrosion resistance of SMSS according to its great effect on pitting resistance equivalent number (PREN = %Cr + 3.3%Mo + 30%N) [10], which is applicable only when N is in solution condition, Cr₂N will be formed during tempering above the temperature required for obtaining reversed austenite [11], impairing not only the toughness but also corrosion resistance of SMSS due to the depletion of Cr in the vicinity of Cr rich nitrides to levels below that required to form the protective oxide layer [12]. In order to prevent the precipitation of Cr₂N during tempering, microalloying elements Nb and V additions can be made to preferentially combine with dissoluble N to form interstitial compounds of microalloying elements during heat treatment process as Nb and V have higher affinity for N than Cr, finally improving the resistance to pitting corrosion in addition to enhanced strength. Based on this concept, a commercial SMSS containing 0.06 wt% N, low Nb (0.025 wt%) and high V (0.09 wt%) has been designed. The exact microstructure and properties performance of this SMSS has not been reported yet. The objective of present study is to investigate the effect of high V (0.09 wt%) and low Nb (0.025 wt%) addition to high nitrogen (0.06 wt%) martensitic stainless steel on the microstructure and properties of a normalized and tempered commercial SMSS.

Section snippets

Experimental procedure

The chemical compositions of the commercial steel are measured by spark emission spectrum and summarized in Table 1, in comparison with those of the other two reference laboratory SMSSs [13]. The ingot was hot rolled at 1200 °C into plates with 12 mm thickness. Normalizing was carried out at 1100 °C for 0.5 h, followed by air cooling to room temperature and tempering heat treatment when the steels were held for 2 h at different temperatures that ranged from 550 °C to 700 °C. After tempering, the

Microstructure characterization

Fig. 1(a) shows the phase stability diagram of Fe–13%Cr–5%Ni– 0.03%C alloy as a function of nitrogen content calculated using Thermo-Calc software and thermo-dynamic database. N is found to effectively expand austenite field. In the absence of microalloying elements additions, thermodynamic potential for precipitation of Cr₂N occurs in high nitrogen steels. Fig. 1(b) shows the effect of microalloying additions of M(Nb and V) on the precipitation potential and on the phase stability diagram of

Discussion

The lath martensite microstructure is achieved even after air cooling from austenite field due to high hardenability of the SMSS used in present study. The prior austenite grains are subdivided into packets of thin laths of martensite. The individual martensite lath is about 0.2–0.3 μm thick and has high dislocation density and the laths within a given packet are in close crystallographic alignment, exhibiting small misorientations (<6°).

According to quantitative analysis of precipitation

Conclusions

1.
Adding low Nb with high V to a commercial high interstitial (0.06%N) SMSS promotes the precipitation of carbo-nitrides enriched in Nb and V in as normalized state since the solution treatment temperature is 1100 °C which is lower than the start temperature of precipitation. Tempering treatment promotes further fine precipitates at the inter lath boundaries and within laths, which can effectively increase the strength, and clustered coarse precipitates due to the high thermodynamic potential and

Acknowledgments

We acknowledge with grateful thanks (i) funding support by CBMM, Brazil, (ii) the award of a scholarship by China Scholarship Council for the student Xiaoping Ma to do research at McMaster University, (iii) help with steels and preparation of materials by Chinese Steel Companies. We wish to thank sincerely Dr. X. Wang, Dr. Carmen Andrie and Dr. Glynis de Silveira for helping with TEM studies, Mr. W. Gong for the assistance with XRD analysis, and Mariana Perez and Marcos Stuart of CBMM for

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Microstructure and properties of 13Cr5Ni1Mo0.025Nb0.09V0.06N super martensitic stainless steel

Abstract

Highlights

Introduction

Section snippets

Experimental procedure

Microstructure characterization

Discussion

Conclusions

Acknowledgments

Mater. Sci. Eng. A

Mater. Sci. Eng. R

Mater. Des.

Mater. Sci. Eng. A

Scr. Mater.

Mater. Charact.

Corros. Sci.

Corros. Sci.

Proc. Conf. Supermartensitic Stainless Steels

Proc. Conf. Supermartensitic Stainless Steels

Proc. Conf. Supermartensitic Stainless Steels

Proc. Conf. Supermartensitic Stainless Steels