Influence of refined hierarchical martensitic microstructures on yield strength and impact toughness of ultra-high strength stainless steel

doi:10.1016/j.jmst.2020.04.001

Abstract

Abstract:

The hierarchical martensitic features in ultra-high strength stainless steel (UHSSS), including the prior austenite grains, martensite packets, blocks and laths with the descending size, were refined to various extents by employing different thermomechanical processes and then carefully characterized. Their relation to yield strength and impact toughness was analyzed. We conclude that the refinement of martensitic structures could lead to the significant increase of yield strength, which follows the Hall-Petch relation with the effect grain size defined by high angle boundaries (HABs). Impact toughness of UHSSS depends on the frequency and capability for retained austenite (RA) grains at both HABs and martensite lath boundaries to trap the propagating cracks via strain-induced transformation, in which the film-like RA grains at lath boundaries appear to make the greater contribution.

Key words: Ultra-high strength stainless steel, Martensite, Yield strength, Impact toughness, retained austenite

Haiwen Luo, Xiaohui Wang, Zhenbao Liu, Zhiyong Yang. Influence of refined hierarchical martensitic microstructures on yield strength and impact toughness of ultra-high strength stainless steel[J]. J. Mater. Sci. Technol., 2020, 51: 130-136.

Figures/Tables 7

Fig. 1. The typical EBSD misorientation and boundary mapping on the hierarchical martensitic microstructure in the studied UHSSS (a) and the illustration for its multi-level microstructural units (b).

Fig. 2. Hierarchical martensitic microstructures on HR75, HR50, HR50-2 and HR75-2 specimens. (a) The prior austenite grains examined by OM; (b) martensite packets examined by SEM; (c) martensite blocks examined by EBSD; (d) martensite laths examined by TEM.

Table 1 The hierarchical martensitic microstructural features, including the average sizes of prior austenite grains (da), martensite packets (dp)/blocks (db)/laths (dl) and the aspect ratio of martensite lath, and mechanical properties, including both Charpy U-Notch impact toughness (CUN) and yield strength (0.2 % offset proof stress, YS), were all measured on UHSSS specimens subjected to four different manufacturing processes

Sample No.	Hot rolling Process	d_a/μm	d_p/μm	d_b/μm	d_l/μm	Aspect ratio	CUN/J	YS /MPa
HR75	75% reduction and finished at 1050?°C	6.3?±?3.7	2.8?±?1.1	2.1?±?1.0	0.24?±?0.11	11.6	186?±?3	1065?±?4
HR50	50% reduction and finished at 1050?°C	14.5?±?8.8	6.3?±?2.7	2.1?±?0.9	0.26?±?0.12	24.2	193?±?3	1035?±?2
HR50-2	50% reduction and finished at 1080?°C	16.1?±?9.1	6.7?±?2.2	2.2?±?0.9	0.25?±?0.11	21.9	190?±?5	1020?±?2
HR75-2	HR75 reheated to 1080?°C for 1?h	76.1?±?45.3	24.0?±?10.6	2.7?±?1.0	0.44?±?0.18	56.6	125?±?1	913?±?3

Fig. 3. X-ray diffraction spectra recorded on HR50, HR75, HR75-2 specimens.

Fig. 4. Plots of yield strength vs. the reciprocal square root of (a) prior austenite grain size (da) and packet size (dp), (b) block width (db) and lath width (dl) as well as (c) the size defined by high angle boundaries (dHBs). All of them are fitted by Hall-Petch equation with the correlation coefficient, R.

Fig. 5. SEM images on the impact fracture surface for HR75-2 (a), HR75 (b) and HR50 (c) specimens; the distributions of dimple sizes in three specimens are shown in (d).

Fig. 6. EBSD misorientation and boundary image mapping on the propagating cracks in HR50 (a) and HR75 (b); Bright-field TEM image and selected area electron diffraction of film-like retained austenite in HR50 (c); EBSD image quality and phase mapping on the matrix of HR50 without cracks (d), near cracks (e) and on HR75 (f) and HR75-2 (g) both without cracks. The green represents FCC phase. Constituents of high angle boundaries vary in specimens having the different PAGS (h).

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