H. R. Yang

Effects of austenitizing treatments and inclusions on secondary hardening and fracture behavior for high Co–Ni steels containing W

Abstract When the inclusions were modified by additions of La and Mn in the WCrCoNi steel, La and Mn sulfides were formed, respectively. When no attempt to modify inclusions was performed, Cr sulfides were formed. While the secondary hardening peak was investigated at 510°C in the 1200 and 1000°C austenitizing conditions, it was shifted down to 475°C in the 899–816°C double-austenitizing condition. The aging acceleration in the latter condition of low-temperature austenitizing correlated with the presence of undissolved carbides containing great amounts of W, increasing the Cr/W ratio in the matrix. In the as-quenched condition, the fracture surfaces of impact specimens exhibited the transgranular dimple mode. While the combined action of impurities and coarse cementite at the grain boundaries leading to the intergranular fracture decreased the impact toughness in the underaging condition, however, the dissolution of cementite eliminated the intergranular fracture and recovered the impact toughness in the peak-aging condition in spite of an increase in hardness. In the 1000°C austenitizing and peak-aging condition, the impact toughness to hardness ratio was relatively high in the presence of large, spheroidal La sulfides with the large interparticle spacing. Its ratio was lowered in the presence of small, spheroidal Cr sulfides and large Mn sulfides of rod shape, due to the small interparticle spacing and the easy stress concentrations at the interfaces, respectively.

Effect of alloying additions on fracture behavior of Mo-containing secondary hardening steels

The effect of Cr, Co, and Ni additions on the secondary hardening behavior in Fe-C-Mo steels was systematically analyzed in terms of their effect on M{sub 2}C carbide formation and reported elsewhere in this journal. In this study, the fracture behavior has been analyzed for the same alloy systems. Impact toughness was determined as a function of aging temperature.

Influences of Severe Deformation and Alloy Modification on Secondary Hardening and Fracture Behavior

Secondary hardening and fracture behavior in the high Co-Ni steels containing W, has been studied in terms of severe deformation, Cr addition, and austenite condition. Two kinds of Co- Ni steels, containing the only W(W steel), and the W and Cr(WCr steel), were severely rolled and followed by direct quenching(DQ). In comparison with undeformed specimens, reaustenitizing( RA) was performed. DQ specimens exhibited higher impact toughness as well as higher hardness, as compared to RA specimens. The aging kinetics of the WCr steel was accelerated due to the Cr addition. While the W steel showed the intergranular embrittlement in the high temperature RA condition, in addition, the WCr steel exhibited the transgranular mode of mostly dimple type.

Influence of Severe Accumulative Rolling in a Low Carbon Microalloyed Steel

Effect of the severe deformation by multi-pass rolling on microstructure and tensile properties was analyzed in terms of rolling temperature, plate thickness, and cooling rate for a modified API X65 steel containing B. The plates, 80 and 50 mm thickness, were rolled six times by 20%/pass (total 75%) to 20 and 12 mm, at 1023 K of unrecrystallized γ region or 973 K of intercritical (α+γ) region, and then quenched in water or oil. All specimens except one oil-quenched condition showed relatively high UTS 700-830 MPa and the continuous yielding(YR～0.6), typical mode of the (ferrite + martensite (bainite)) dual phase microstructure. In contrast, one oil-quenched specimen with the 973 K-20 mm condition, exhibited the discontinuous yielding (YR～0.8), indicating that the microstructure basically consists of ferrite plus pearlite, as well as a relatively low UTS 660 MPa. The degree of deformation really occurring within materials, i.e., strain hardening seems to be enhanced with a decrease in deformation temperature. As the degree of deformation increases, the remaining austenite, not dynamically transformed to fine ferrite, becomes increasingly unstable. A lower hardenability of this remaining austenite thus would lead to a higher possibility to transform into the (ferrite + pearlite) structure of lower strength rather than the (ferrite + martensite (bainite)) of higher strength.

Mechanical Properties in Ultrahigh Strength Secondary Hardening Steels Bearing Co and Ni

Applying the severe deformation, the mechanical properties related to secondary hardening were investigated. The Mo-Cr-Co-Ni steels containing (5-13)wt% Co and (8-14)wt% Ni were severely rolled at 850 °C, followed by direct quenching(DQ). DQ specimens were then isothermally aged at 475°C. Ni additions promoted aging kinetics, accompanied by a little enhancement in hardness. In contrast, Co additions enhanced the peak hardness, but did not induce a remarkable aging acceleration. In the 13Co addition group, an actual secondary hardening, that is, the peak hardness is higher than the as-quenched hardness. Variation in peak hardness(Rc) in alloys is summarized as follows; 14Ni-13Co(57.2) ≒ 11Ni-13Co(57.1) > 8Ni-13Co(56.5) > 11Ni- 9Co(56.2) > 8Ni-9Co(54.4) > 11Ni-5Co(53.4).

Effect of Multipass Severe Rolling Process in the API X65 Steel

In the API X65 steel, effects of rolling and cooling conditions on microstructure and mechanical properties were studied. In the case of accelerated cooling after multi-pass rolling in the high/low unrecrystallized range, the tensile strength was 574-670 MPa and the impact toughness was 74-109 J. In the case of accelerated cooled to 550°C and then interrupted by air cooling, on the other hand, those values were 524-538 MPa and 100-135 J, respectively. Whereas the former exhibited the continuous yielding, the latter showed discontinuous yielding. In addition, yield ratio increased from 0.59-0.67 to 0.85-0.87, accompanied with the enhancement of yield strength. Ultrafine ferrite grains formed by the strain induced dynamic transformation during the severe rolling and second phases formed during cooling were observed. In accelerated cooling and interrupted cooling conditions, main second phases formed after cooling were martensite and pearlite, respectively. Separation cracking mostly observed at interfaces of ferrite matrix and second phases, may be attributed to the intrinsic interfacial weakness.

On Carbide Particle Reinforced Al Composites Fabricated by Pressureless Infiltration Technique

5052 Al matrix composites reinforced with carbide particles (SiC, TiC, and B4C) were fabricated by the pressureless infiltration and the tensile properties were analyzed. The strength values in the control Al were significantly increased over those of the commercial alloy while the strain to failure of the former decreased. Strength values in the composite reinforced with carbide particles were further increased compared to the control alloys. It was observed that strengthening effect by an addition of reinforcement varied with according to reinforcement types. By relative comparison, both TiC and B4C particles may be effective reinforcement compared to SiC particles in Al matrix composites.