Shuqi Wang

Alloying Design for High Wear-Resistant Cast Hot-Forging Die Steels

The alloying design of cast hot-forging die steels was analyzed. The relationship of the life of cast hot-forging dies with the failure patterns was studied. The thermal wear resistance was believed to be the key property for the alloying design of cast hot-forging die steels. The alloying design parameters were selected and optimized for the cast hot-forging die steel with high wear resistance. The wear resistance of the optimized cast die steel was evaluated in comparison with commercial H13 steels and 3Cr2W8V steel. In the new cast hot-forging die steel, VC is predominant carbide with Cr and Mo as the main solution elements in α-Fe. It is found that the cast die steel has significantly lower wear rate than normal H13 steel and 3Cr2W8V steel, almost the same as that of high purity H13 steel. The high wear resistance of the new cast hot-forging die steel can be attributed to its reasonable alloying design and nonsensibility to the detrimental function of S and P.

Effect of Microstructures on Elevated-Temperature Wear Resistance of a Hot Working Die Steel

Elevated-temperature wear tests under atmospheric conditions at 400 °C were performed for a hot working die steel H21 on a pin-on-disk wear tester. The phase and morphology of worn surfaces were examined using XRD and SEM, and the relation of wear resistance to tempered microstructures was studied for H21 steel. XRD patterns exhibit that oxidative wear is a predominated wear mechanism with Fe3O4 and Fe2O3 on worn surfaces. It is found that with increasing normal load, obvious plastic deformation of substrate appears on worn surfaces. Microstructures start to affect apparently wear resistance of the steel with an increase of load. Under loads of 50–100 N, wear losses of steel retain low values and relatively approach for steels with various microstructures. As loads are increased to 150–200 N, wear losses of steel start to increase obviously and present apparent difference for steel with various microstructures. Wear resistance is found to increase in the sequence as follows: tempered sorbite, tempered martensite, tempered troostite without secondary hardening and tempered troostite with secondary hardening or upcoming one. Higher strength and microstructural stability are required for steels with excellent wear resistance.

Selection of Heat Treatment Process and Wear Mechanism of High Wear Resistant Cast Hot-Forging Die Steel

Dry sliding wear tests of a Cr-Mo-V cast hot-forging die steel was carried out within a load range of 50–300 N at 400 °C by a pin-on-disc high-temperature wear machine. The effect of heat treatment process on wear resistance was systematically studied in order to select heat treatment processes of the steel with high wear resistance. The morphology, structure and composition were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS); wear mechanism was also discussed. Tribo-oxide layer was found to form on worn surfaces to reduce wear under low loads, but appear inside the matrix to increase wear under high loads. The tribo-oxides were mainly consisted of Fe3O4 and Fe2O3, FeO only appeared under a high load. Oxidative mild wear, transition of mild-severe wear in oxidative wear and extrusive wear took turns to operate with increasing the load. The wear resistance strongly depended on the selection of heat treatment processes or microstructures. It was found that bainite presented a better wear resistance than martensite plus bainite duplex structure, martensite structure was of the poorest wear resistance. The wear resistance increased with increasing austenizing temperature in the range of 920 to 1120 °C, then decreased at up to 1 220 °C. As for tempering temperature and microstructure, the wear resistance increased in following order: 700 °C (tempered sorbite), 200 °C (tempered martensite), 440 to 650 °C (tempered troostite). An appropriate combination of hardness, toughness, microstructural thermal stability was required for a good wear resistance in high-temperature wear. The optimized heat treatment process was suggested for the cast hot-forging steel to be austenized at 1020 to 1120 °C, quenched in oil, then tempered at 440 to 650 °C for 2 h.

In Situ Synthesis of Heat Resistant Gradient Composite on Steel Surface

A heat resistant gradient composite was synthesized in situ on steel with the self-propagating high temperature synthesis (SHS) reaction of 3Ni-Al-Ti-C system during casting. The phases, microstructure, and composition of the composite were analyzed by using an X-ray diffractometer (XRD), and a scanning electron microscope (SEM) coupled with an energy-dispersive X-ray spectroscope (EDS). The formation mechanism of the composite is also discussed. TiC/Ni3 Al/steel gradient composite is achieved by forming the gradient distributions of Fe, Ni, and Al, accompanied with the gradient variation of the microstructure from TiC/Ni3 Al, to TiC/Ni3 Al/steel, and to steel. The composite is in situ synthesized through whole reaction of 3Ni-Al-Ti-C system in liquid steel and densification procedure, and the liquid steel infiltrates into pores in the SHS product and forces liquid Ni3 Al to form self-compaction further.

Wear Behavior and Mechanism of Spheroidal Graphite Cast Iron

Wear behavior and mechanism of spheroidal graphite cast iron were studied on a pin-on-disk elevated temperature wear tester. The phase and morphology of worn surfaces were examined by X-ray diffraction and scanning electron microscopy. Results show that with an increase of load, wear rate of spheroidal graphite cast iron gradually increases under low loads, rapidly increases or potentially increases under high loads; wear rate increases with increasing ambient temperature. At 25–200 °C, adhesive wear prevails; oxidative wear and adhesive wear coexist at 400 °C. As load surpasses 150 N at 400 °C, extrusive wear appears. The elevated-temperature wear of spheroidal graphite cast iron is a physical and chemical process including the following reactions: xFe + y/2O2—FexOy, 2C+ O2—2CO and FexOy +yCo—xFe+yCO2. Hence, at 400 °C, the amount of graphite and tribo-oxides are substantially reduced because of reductive function of graphite. It can be suggested that wear-reduced effect of graphite and tribo-oxides is impaired.

Dry sliding wear behavior and mechanism of AM60B alloy at 25–200 °C

Abstract Dry wear tests under atmospheric conditions at 25–200 °C and loads of 12.5–300 N were performed for AM60B alloy. The wear rate increases with increasing the load; the mild-to-severe wear transitions occur under the loads of 275 N at 25 °C, 150 N at 100 °C and 75 N at 200 °C, respectively. However, as the load is less than 50 N, the wear rate at 200 °C is lower than that at 25 °C or 100 °C. In mild wear regimes, the wear mechanisms can be classified into abrasive wear, oxidation wear and delamination wear. Delamination wear prevailed as the mild-to-severe wear transition starts to occur; the delamination occurs from the inside of matrix. Subsequently, plastic-extrusion wear as severe wear prevails accompanied with the transition. The thick and hard tribo-layer postpones the mild-to-severe wear transition due to restricting the occurrence of massive plastic deformation of worn surfaces.

Effect of artificial tribological layer on sliding wear behavior of H13 steel

An artificial tribological layer was formed on the worn surface during sliding, through supplying MoS2, Fe2O3 or their equiponderant mixtures onto the sliding interface of H13/GCr15 steels. The effect of this tribological layer on the wear behavior of H13 steel was studied. The worn surfaces and subsurfaces of H13 steel were thoroughly characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) the wear mechanisms were explored. The research results demonstrated that tribological layer did not exist during sliding of H13 steel with no additive, but it formed with the addition of MoS2, Fe2O3 or their equiponderant mixtures. When there was no tribological layer, the wear rate rapidly increased with an increase of the load. In this case, adhesive and abrasive wear prevailed. As the additives were supplied, the artificial tribological layer was observed to be immediately formed and stably existed on worn surfaces. This tribological layer presented an obvious protective function from wear and friction. Hence, the wear rate and friction coefficient were significantly decreased. M0S2 as tribological layer seemed to present more obvious protective function than Fe2O3. By supplying their mixture, the artificial tribological layer possessed not only the load-carrying capacity of Fe2O3, but also the lubricative capacity of MoS2. These two simultaneous capacities could improve the friction and wear properties of H13 steel further.

Effect of nanoparticles on the tribo-layers and the tribology of a steel-on-steel couple:

The tribological behavior and tribo-layers of AISI 1045 steel sliding against 52100 steel were investigated in the case of supplying MoS2, Fe2O3, and their mixtures onto the sliding interface. When nanoparticles were supplied, tribo-layers were formed on the worn surfaces. The tribological behavior of the sliding pair depended on the characteristics of tribo-layers, which were decided by different nanoparticles. As the additives—especially the ones containing MoS2—were supplied onto the sliding interface, the wear rates and friction coefficients of both 1045 steel and 52100 steel were markedly decreased to extremely low values, approaching zero and marginally undulated with the increase in load. Single-component Fe2O3 nanoparticles markedly reduced the wear rate of 1045 steel with slightly increased friction coefficient, but its decreased extent was merely half of that of the additives containing MoS2. The improvement of the tribological performance of steels was attributed to the formation of protective tribo-layers. The addition of pure Fe2O3 resulted in the formation of insert-type tribo-layers, while cover-type tribo-layers were formed by the addition of the mixture additives of Fe2O3+MoS2 and pure MoS2. The cover-type tribo-layers provided more protective and lubricative functions than that of the insert-types.

Comparative research on the effect of an oxide coating and a tribo-oxide layer on dry sliding wear of Ti–6Al–4V alloy

The effect of an oxide coating and a tribo-oxide layer on dry sliding wear of Ti–6Al–4V alloy was comparatively studied. The oxide coating was prepared on Ti–6Al–4V alloy by a thermal oxidation/diffusion process; the tribo-layer was an in situ produced mechanically mixed layer during dry sliding. The oxide coating markedly improved the wear performance of Ti–6Al–4V alloy at room and elevated temperatures. Tribo-layers were classified into three types: no-oxide tribo-layer, porous, and dense tribo-oxide layers. Both porous and dense tribo-oxide layer presented protective function, thus significantly improving wear performance of Ti–6Al–4V alloy. However, no other than dense tribo-oxide layer was qualified to be comparable to the oxide coating, which almost possessed the same wear-reduced function as the oxide coating. Even when the oxide coating was severely delaminated, the tribo-oxide layer would replace the oxide coating and took effect to protect from wear.

Dry Sliding Wear Behavior of Hot-Dip Aluminized Ti-6Al-4V Alloy as a Function of Sliding Velocity

An aluminized coating was prepared on Ti-6Al-4V alloy by hot-dip aluminizing and subsequently diffusion treatment. Dry sliding wear tests were performed for the aluminized and uncoated Ti-6Al-4V alloy under the loads of 10-50xa0N at the sliding velocities of 0.5-4xa0m/s. The wear resistance of the titanium alloy was improved by the aluminized coating under various conditions, especially at 4xa0m/s. The improved wear performance was suggested to be attributed to the Ti-Al coating and tribo-oxide layer. Tribo-layers were identified to form on worn surfaces under various conditions but their influence on the wear behavior and mechanism was decided by the amount and kind of oxides. The outmost values of the microhardness distribution at subsurfaces as a function of load could be used to identify the property and stability of tribo-layers. At 4xa0m/s, oxide-containing tribo-layers (more TiO, TiO2 and trace Fe2O3) presented high hardness and stability, thus possessed an obvious wear-reduced function. Conversely at 2.68xa0m/s, no-oxide tribo-layers did not show the protection from wear because of their lower hardness and instability. The formation of tribo-layers under various sliding speeds was noticed to be a process including wear debris production, oxidation and accumulation and densification, even sintering.