SURFACE STRENGTHENING TECHNOLOGY FOR MECHANICAL PARTS

This paper has summarized five surface strengthening methods, and these methods could to improve the surface properties of materials. The selection of mechanical parts materials has determined according to their working conditions. The work-piece in using cannot avoid defects in the material. This paper has introduced surface deformation enhancement, surface phase transformation enhancement, ion implantation technology, surface diffusion and infiltration technology, chemical transformation technology and surface coating technology. And has also included the principle of every surface technology, various technologies, parameters, strengthening characteristics, as well as strengthening effect and matters needing attention. The hardness, residual stress and corrosion resistance off mechanical parts could be improved through these surface strengthening methods. It is convenient to find the strengthening method and parameters in this paper when strengthening mechanical parts.

Wear Resistance Research of Advanced High Strength Steels

The wear performance and wear mechanism under two-body abrasion of five advanced high strength steels, i.e. Nanobainite (NB) steel, Tempered Martensitic (TM) steel, Dual Phase (DP) steel, Transformation Induced Plasticity (TRIP) Steel and Twining Induced Plasticity (TWIP) steel were studied. By using the scanning electron microscopy (SEM), we investigated the wearing surface. Phase transformation strengthening behavior was also be discussed by analyzing the surface and sub-surface after abrasion. The results showed that micro-cutting was the major role of wear mode in the condition of two-body abrasion. In the circumstance of two-body abrasion, hardness was an important factor, the property of wear resistance enhanced while the hardness increased except for TM steel. NB steel possessed the best wear resistance which was 1.71 times higher than that of TWIP steel. The retained austenite transformed into martensite which can improve the hardness so that it enhanced the wear resistance of NB steel.

Austenite-Martensite Phase Transformation of Biomedical Ni50.8Ti49.2 Alloy by Ball Burnishing

Nitinol alloys have received considerable attentions in biomedical and aerospace applications. They can exhibit both austenite and martensite phases at room temperature. Austenite can transform to martensite by applied stress or temperature. Ball burnishing is a very promising technique to modify surface integrity via plastic deformation on the workpiece surface. Phase transformation of Nitinol by burnishing may occur at certain load, which results in the mechanical property change on the workpiece. A burnishing experiment has been conducted in this research at different burnishing loads. The burnishing tracks are characterized and microstructures in the subsurface are studied. A corresponding simulation is also performed to shed light on phase transformation mechanism of Nitinol in burnishing.

Surface Phase Transformation Analysis of Nanozirconia Toughened Alumina Ceramics under Two-Dimentional Ultrasonic Vibration Grinding

Based on good processing property of two-dimentional ultrasonic vibration assisted grinding (TUVAG), phase transformation in the grinding surface of nanozirconia toughened alumina ceramics (ZTA ceramics) is studied by XRD analysis. Experimental results show different processing method and grinding depth may lead to surface phase transformation of ZTA ceramics. Under the same grinding parameters, phase transformation ratio of grinding surface under TUVAG is higher than that under diamond grinding (DG), and its phase transformation ratio increases with the increasing of grinding depth. Phase transformation ratio can effectively inhibit generation and expansion of microcracks of grinding surface.

Selective Oxidation and Sub-Surface Phase Transformation during Austenitic Annealing of TWIP Steels

The selective oxidation of Al-free and Al-added Twinning Induced Plasticity (TWIP) steels during full austenitic annealing at 800°C in N2+10%H2 atmosphere at a dew point of -17°C was investigated by means of HR-TEM of FIB cross-sectional samples. For Al-free TWIP steel, a dense MnO layer was formed on the surface. Crystalline c-xMnO.SiO2(x2) particles and amorphous a-xMnO.SiO2(x<0.9) particles were found at the MnO layer/steel matrix interface. In the subsurface, Mn depletion resulted in the transformation of the austenite to the ferrite phase in a narrow zone. For Al-added TWIP steel, a continuous outer MnO layer and a transition layer consisting of amorphous a-xMnO.SiO2(x<0.9) and crystalline c-MnO.Al2O3(0.8<x<1.2) were formed. The interface between the a-xMnO.SiO2(x<0.9) and c-MnO.Al2O3(0.8<x<1.2) layers had rough structure and 20~50nm diameter voids were formed at the interface. Meanwhile, a narrow Mn-depleted ferrite layer was also formed in the subsurface. The void formation is very likely related to Kirkendall effect occurring during the oxides formation. The thick MnO layer and the voids constitute major challenges to the successful hot dip galvanization of TWIP steels in industrial HDG lines.