Li Dianzhong

PRECIPITATION BEHAVIOR OF M23C6 AND ITS EFFECTS ON DUCTILITY AND TOUGHNESS OF A NOVEL Cr－Mn－N AUSTENITIC HEAT RESISTANT STEEL

Cr-Mn-N austenitic heat resistant steels have bright future in wide applications due to their lower cost, excellent mechanical properties, and outstanding local corrosion resistance after substituting the nickel partly or completely by the Mn, N and C elements. As the content of C or N elements increasing in Cr-Mn-N steels, a large quantity of precipitates form during hot working, resulting in embrittlement of these steels. In the present study, a new Cr-Mn-N austenitic heat resistant steel was aged at temperatures from 600 to 1000 in duration from 10 min up to 6000 min. By OM, SEM and XRD, the microstructure evolution of precipitates and their effects on ductility and toughness of the studied steel were investigated. The results indicated that precipitates formed during aging were mainly Cr-rich M23C6 carbides, whose morphologies changed in a sequence of intergranular films, lamellae in cellular microstructure and intragranular rods or particles with the increase of aging time and/or temperature. The time-temperature-precipitation (TTP) curves for M23C6 carbides were determined, which have a typical C-shaped profile with a nose temperature between 850 and 900 , and an incubation period not more than 1 min. In addition, it was found out that the aging embrittlement of the studied steel is strongly dependent on the morphology of M23C6 carbides. The intergranular film-shaped M23C6 carbides were considered as the main factor to favor cracks rapidly expanding along grain boundaries, finally resulting in brittle intergranular fracture. KEY WORDS Cr-Mn-N austenitic heat resistant steel, M23C6 carbide, cellular microstructure, aging embrittlement

STUDY ON VOID HEALING BEHAVIOR DURING FORGING PROCESS FOR 25Cr2Ni4MoV STEEL

Based on the measured stress-strain curves and thermo-physical data of 25Cr2Ni4MoV steel, a FEM model of void closure in heavy forging process was established through ABAQUS software. The void closure behaviors under different hot plasticity conditions have been investigated. The FEM results showed three distinct stages during the void closure. The voids with the same position in a cylindrical specimen close up at a similar height reduction ratio (ΔH/H0) around 25% at different temperatures, which indicates that the void closure is not sensitive to the deformation temperature. On basis of the FEM results, a physical-simulated experiment for void bonding process has been performed through compression tests on a hollow cylindrical specimens, with deformation temperatures ranging from 900 to 1200 ,Δ H/H0 from 25% to 45% and a constant strain rate 0.01 s −1 .T he experimental results have shown that the ΔH/H0 for the void completely bonded at 1200, 1100 and 1000 are all about 35%, but increases to 45% when the deformation temperature decreased to 900 ,w hich confirms that void bonding is a diffusion controlled process. Through the experimental results, it can be further demonstrated the high temperature combined with severe deformation can enhance the ability of atoms transition and decrease the micro-gap between contacted surfaces. Finally, the influencing factors on the bonding efficiency were investigated through SEM observation on the fractured surface of the tensile specimen with internal closed void.