The Study On Microstructure And Fatigue Properties Of Plasma Transferred Arc (PTA) Hardening On Grey Cast Iron

During the last years, the materials demand gave rise to an important improvement inthe techniques to modify surface. High energy beam alloying like laser, electron beam andplasma transferred arc (PTA) is one of the techniques, which has been generally used.Although the hardness, wear resistance, erosion resistance could be increased by selectingappropriate alloying material and optimizing processing parameters, there was a commonproblem existed that the alloying represented a state of high residual stress, low toughnessand poor ductile. For industrial-utilized components which served in the life range from102to106cycles, fatigue property after alloying should be highly considerate.In the present work, we applied PTA to alloy TiC-Cr-W powder into the matrix indifferent parameters to change not only the composition and the microstructure of the graycast iron but also the fatigue resistance. The fatigue properties of the PTA alloyingTiC-Cr-W were compared with the PTA hardening without adding the reinforcements andmatrix. Further, we carried different tempering temperatures on PTA alloying TiC-Cr-Wwith an aim to explore for a close control of the microstructure and fatigue property. Theeffort was focused on calculating phase volume fraction variation of alloying zone as wellas the microstructure transformation of heat affected zone. In addition, the effect oftempering on fatigue life was studied by Weibull distribution plots.This paper’s main reserch results are as follows:1) The hard layer produced by PTA alloying could be divided into three different regions:AZ, HAZ and the matrix. The microstructure of AZ shows that it consists of primaryaustenite, martensite, a eutectic of (Fe,Cr)7C3carbide and austenite as well as theuniformly distributed reinforcement. Martensite was only formed at the HAZ. TheXRD, EDAX and TEM tests confirms that the reinforcing particle at the AZ is the un-melted TiC. The microhardness of PTA strengthening reveals a gradient variation,and the maximum values of PTA alloying TiC-Cr-W, PTA hardening and matrix are1069,856and269HV0.2respectively. TiC-W-Cr powder increases the hardenability ofgrey cast iron. The surface morphology, microstructure and microhardness distributionis related to the processed parameters.2) Weibull plots on fatigue life illustrates that while PTA hardening withoutreinforcement shows the lowest Na, PTA alloying TiC-Cr-W improves the Naof thematrix by64%. On the other hand, after PTA treatment, the shape parameter ofWeibull distribution deceases, suggesting that the fatigue life becomes much scattered.The deflects of cast iron are one of the main attribution to scatter of fatigue life. Fromthe observation in SEM, both alloying zone (AZ) and matrix was showed a cleavagefracture. However, AZ becomes much more flattered after PTA treatment, showing thatthe striation and quasi-cleavage feature is the dominant mechanism for fatigue crackgrowth. The matensite at HAZ usually breaks at the original austenite boundary.Therefore, the Heat affected region (HAZ) shows an obvious intergranular fracture.3) PTA alloying TiC-Cr-W has a retarding effect on the fatigue crack growth rate whenthe ΔK is low. However, when ΔK exceeds a certain degree, this effect disappears andfatigue crack growth rate accelerates. PTA alloying TiC-W-Cr eliminated the stressconcentration at the edge of graphite and produced hard carbide, resulting in frequentcrack deflection.4) With the tempering temperature increasing, the tempered martensite was formed in theaustenite cell, and the austenite was refined accompanied by transformation from(Fe,Cr)7C3to (Fe,Cr)23C6as well as dissolution of TiC. At HAZ, the microstructurePTA alloying produced was mainly composed of martensite with some un-meltedgraphite. After the tempering temperature increased, martensite gradually decomposedwith the in situ the formation of coarser temper pearlitic structure. X-ray diffractionsuggested that the size effect and the strain effect were eliminated. 5)473K tempered after PTA alloying shows the highest microhardness; however, themicrohardness decreased after temperature further increased. While473K temperingcould significantly improve the fatigue life of alloying specimens, further elevation ofthe temperature directed fatigue strength degradation. And tempering treatmentimproved distribution of fatigue life, making it less dispersed.