Xiaochun Wu

Effect of titanium on the microstructure and hardness uniformity of non-quenched and tempered prehardened steel for large-section plastic mould

Two non-quenched and tempered (NQT) prehardened plastic mould steels, differing by titanium content, were produced to have good hardness uniformity in large sections (485 × 970 and 465 × 1325 mm2). The effect of titanium on continuous cooling transformation curves, hardness uniformity and microstructure (grain size, phase constituents) were studied. It was found titanium had refined the grain size of NQT steel by the pinning effect of Ti (C, N) at grain boundaries, and titanium also effectively postponed the nucleation of proeutectoid ferrite in intercritical region. Using finite element method, the cooling rate from core to surface along longitude direction of the steels was predicted. The results indicated that the NQT steel with titanium had better process adaptability than that of NQT steel without titanium. Compared with production of NQT steel without titanium, production for NQT steel with titanium can largely save the production cost by shortening the production cycle and reducing the resources.

Thermomechanical Analysis of Deep Cryogenic Treatment of Navy C-Ring Specimen

A thermomechanical coupling numerical model is built to reproduce the deep cryogenic treatment (DCT) of a cold work die steel Cr8Mo2SiV (SDC99) Navy C-ring specimen and explore the transient temperature distribution and microstructure evolution in specimen. Furthermore, the predicted results are validated by x-ray diffraction analysis and hardness measurement. The results indicate that for both the quenching treatment (QT) and DCT, the differences in cooling rate and temperature distribution between the gap and core regions of specimen are significant. The gap region of specimen shows a more rapid cooling rate, while the core region of specimen presents a slower cooling rate. There is an underlying risk of hardening crack at the gap region of specimen during the cooling process. Both the cooling rate and the temperature difference that occurred in the DCT process are markedly smaller than that in the QT process. After QT, about 15.5% of austenite will still remain, especially in the edge and corner of specimen, which is a potential factor for component failure. Subjected to DCT, the microstructure distribution of specimen demonstrates a distinct change and finally the volume fraction of retained austenite decreases to about 2.3%, which principally localizes at the gap region of specimen. The hardness of specimen after DCT has been a dramatic increase and shows a uniform distribution. In comparison with the experimental data, the predicted results show a quite good accuracy. It indicates that the thermomechanical couple model employed in this study can be used to optimal control of the DCT process.

Tempering stability of Fe–Cr–Mo–W–V hot forging die steels

The tempering stability of three Fe–Cr–Mo–W–V hot forging die steels (DM, H21, and H13) was investigated through hardness measurements and transmission electron microscopy (TEM) observations. Both dilatometer tests and TEM observations revealed that DM steel has a higher tempering stability than H21 and H13 steels because of its substantial amount of M2C (M represents metallic element) carbide precipitations. The activation energies of the M2C carbide precipitation processes in DM, H21, and H13 steels are 236.4, 212.0, and 228.9 kJ/mol, respectively. Furthermore, the results indicated that vanadium atoms both increase the activation energy and affect the evolution of M2C carbides, resulting in gradual dissolution rather than over-aging during tempering.

Effects of mechanical strain amplitude on the isothermal fatigue behavior of H13

Isothermal fatigue (IF) tests were performed on H13 tool steel subjected to three different mechanical strain amplitudes at a constant temperature to determine the effects of mechanical strain amplitude on the microstructure of the steel samples. The samples’ extent of damage after IF tests was compared by observation of their cracks and calculation of their damage parameters. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to observe the microstructure of the samples. Cracks were observed to initiate at the surface because the strains and stresses there were the largest during thermal cycling. Mechanical strain accelerated the damage and softening of the steel. A larger mechanical strain caused greater deformation of the steel, which made the precipitated carbides easier to gather and grow along the deformation direction, possibly resulting in softening of the material or the initiation of cracks.

Enhancement of mechanical properties for quenched and partitioned steels via intragranular austenite

Mechanical properties of quenched and partitioned steels were dramatically enhanced by two types of austenite, including intragranular austenite in ferrite grains and allotriomorphic austenite. The formation mechanism of intragranular austenite as well as its effect on ductility was studied. Digital image correlation strain analyses combined with microstructural observations indicated that most intragranular austenite experienced higher plastic strain than allotriomorphic austenite upon loading, however, intragranular austenite behaved more stable than allotriomorphic austenite, even if both characteristics were very similar. Intragranular austenite in ferrite played a positive role during deformation, like reconciliation of strain, grain refinement as well as transformation-induced plasticity effect. Further adjustment for the amount and stability of intragranular austenite is expected to enhance the ductility of transformation induced plasticity-assisted steels.

Study of the microstructure and properties of a precipitation-hardened steel-10Ni3MnCuAl block

A study is made of the microstructure, mechanical properties, machinability, and service properties of a 250 × 800 × 2000 mm block of 0.15% C – 1.87% Mn – 3% Ni – Cr – Cu – Al steel after its continuous cooling. The steel has good hardenability and polishability, and a pattern can easily be etched on its surface.