Limitations of the homogenization process of bulk martensitic mold steel-depending on the comprehensive manifestation of various strengthening

doi:10.1016/j.jmst.2021.10.046

Abstract

Abstract:

A homogenization treatment (1250 °C + 12 h) was carried out to minimize the micro-segregation of bulk 718H martensitic mold steel, as verified by advanced experimental characterization and kinetic model of diffusion. However, new research found that there are still limitations in the use of the homogenization process. The result indicates that the chemical heterogeneity can be significantly reduced after homogenization. The segregation ratio of Cr and Mo elements of sample decreased by 40.9% and 35.6% of the original level, respectively. Simultaneously, the test steel with higher strength and toughness is produced by controlling micro-segregation tempered from 540 °C to 650 °C. Importantly, it reveals that the impact energy is increased by up to 27.3%. The isotropy of impact energy in different directions can reach 0.89, resulting in an overall improvement in the isotropy. Toughness mainly depends on the orientation relationship between the crack propagation direction and the band segregation region. The chain carbides formed due to the decomposition of the micro-segregated region during tempering are considered the main source of cracks. The more evenly distributed the subsequent tempered carbides after homogenization, resulting in an increase in toughness. However, an abnormal phenomenon is found in which the yield strength after homogenization is lower than that of the untreated sample tempered at 700 °C. This result can be attributed to the combined influences of precipitation strengthening and fine grain strengthening by analyzing various strengthening mechanisms. The mutually restrictive strengthening effect leads to the limitations of the homogenization process of bulk martensitic mold steel.

Key words: Martensitic mold steel, Strength-toughness, Micro-segregation, Synergistic strengthening

Hanghang Liu, Paixian Fu, Hongwei Liu, Chen Sun, Ningyu Du, Dianzhong Li. Limitations of the homogenization process of bulk martensitic mold steel-depending on the comprehensive manifestation of various strengthening[J]. J. Mater. Sci. Technol., 2022, 117: 120-132.

Figures/Tables 19

Table 1. Chemical composition of the test steel (wt%).

Steel	C	Si	Mn	Cr	Ni	Mo	O
A#	0.34	0.31	1.53	2.02	1.03	0.19	0.0008
B#	0.33	0.30	1.49	2.00	1.05	0.20	0.0009

Fig. 1. (a) heat treatment process of A# ingot, and (b) heat treatment process of B# ingot.

Fig. 2. (a) OM image of as-cast microstructure, (b) SEM image of as-cast microstructure, (c) EPMA line scanning area, and (d-g) the distribution of C, Ni, Cr, Mn, Si and Mo elements.

Table 2. SR of alloying elements of as-cast samples of B# ingot (wt%).

Empty Cell	Cr	Mn	Mo	Si
Microsegregation	2.564	2.395	0.280	0.447
Matrix	1.249	1.149	0.138	0.258
SR	2.05	2.08	2.02	1.73

Fig. 3. (a) Original as-cast microstructure, (b) as-cast microstructure after homogenization, (c) original microstructure after forging and (d) as-forged microstructure after homogenization.

Fig. 4. EPMA element mapping of as-cast microstructure of B# sample.

Fig. 5. EPMA element mapping of microstructure after homogenization of A# sample.

Fig. 6. (a) EPMA line scanning area of as-cast microstructure of A# sample, and (b-e) the distribution of C, Ni, Cr, Mn, Si and Mo elements.

Table 3. SR of alloying elements of as-cast samples of A# ingot (wt%).

Empty Cell	Cr	Mn	Mo	Si
Microsegregation	1.639	1.619	0.246	0.388
Matrix	1.351	1.249	0.189	0.301
SR	1.21	1.29	1.30	1.28

Fig. 7. SEM microstructures of 718H steel with different tempering temperatures: (a) B#, 540 °C, (b) B#, 600 °C, (c) B#, 650 °C, (d) B#, 700 °C, (e) A#, 540 °C, (f) A#, 600 °C, (g) A#, 650 °C, and (h) A#, 700 °C.

Fig. 8. Carbide morphologies evolution of 718H steel with different tempering temperatures: (a) B#, 540 °C, (b) B#, 600 °C, (c) B#, 650 °C, (d) B#, 700 °C, (e) A#, 540 °C, (f) A#, 600 °C, (g) A#, 650 °C, and (h) A#, 700 °C.

Fig. 9. OM micrographs of austenite grain of quenched samples: (a) A# sample and (b) B# sample; EPMA line scanning area of the enriched and diluted regions in B# sample (c) and the distribution of Cr, Mn and Mo elements (d-f).

Fig. 10. EBSD crystallographic analyzes of samples tempered at 600 °C, IPF images: (a) B# sample, (c) A# sample, HAGBs images: (b) B# sample and (d) A# sample.

Table 4. Maximum microhardness in different areas of A# and B# as-cast samples (HV).

Samples	Microsegregation	Matrix	Difference value
B#	709 ± 15	380 ± 15	329
A#	614 ± 15	579 ± 15	35

Fig. 11. Mechanical properties of A# and B# samples tempering at different temperatures: (a) hardness distribution of A# sample, (b) hardness distribution of B# sample, (c) hardness difference between A# and B# samples, (d) impact property, (e) yield and tensile strength and (f) section shrinkage and elongation.

Fig. 12. SEM fracture morphologies and EDS analysis of transverse impact samples with different tempering temperatures: (a) B#, 540 °C, (b) B#, 600 °C, (c) B#, 650 °C, (d) B#, 700 °C, (e) A#, 540 °C, (f) A#, 600 °C, (g) A#, 650 °C, and (h) A#, 700 °C.

Fig. 13. Bright-field TEM micrographs and SAED pattern of tempered precipitates of test steels with different tempering temperatures: (a) A#, 700 °C, and (b) B#, 700 °C.

Table 5. Diffusion coefficients of Cr and Mo elements at 1250 °C.

Temperature ( °C)	D_Cr/m²/s	D_Mo/m²/s
1250	1.03 × 10^-13	2.10 × 10^-14

Fig. 14. Schematic diagrams of microsegregation evolution: (a) as-cast, (b) forged, (c) tempered states; and (d) schematic diagram of crack source analysis of impact sample.

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