Ya Zheng Liu

Effects of Austenitizing Temperature and Cooling Rate on the Mechanical Properties of 36MnVS4 Steel for Automobile Engine Connecting Rod

The relationship between microstructure and mechanical properties of 36MnVS4 steel was studied. Different prior austenite grain sizes were obtained by austenitizing at 850, 950 and 1050 °C, respectively, and different ferrite contents were obtained by different cooling rates. Austenitizing temperature mainly influenced the grain size. With the austenitizing temperature increasing, grain size increased and the phase transformation starting temperature increased. Also, the strength increased and the plasticity and toughness decreased. Cooling rate mainly influenced the microstructure percentage. With the cooling rate increasing, ferrite percentage decreased and pearlite percentage increased. And meanwhile, the strength increased and ductility and toughness decreased. Microstructure had a significant influence on fracture splitting properties. With the grain size increasing, fracture splitting properties were markedly improved. With the ferrite percentage increasing and pearlite percentage decreasing, fracture splitting properties were worsened.

Effect of Niobium Additions on Austenite Grain Coarsening Behavior of GCr15 Bearing Steel

The effect of 0.018% niobium additions on austenite grain coarsening behavior of GCr15 bearing steel was studied. Results indicate that the coarsening temperatures of No.1 and the No.2 experimental steel were 950°C and 1100°C. The austenite grain coarsening temperature was increased by 150°C by the addition of 0.018% Nb in bearing steel. The grain growth equations of two experimental steels at different soaking temperatures from 850°C to 1250°C with the soaking time of 30 min are as follows: the equation of No.1 steel is D1=1.85×105·exp (-6.57×104/RT); the equations of No.2 steel below and above 1100°C is D2=5.08×102·exp (-2.49×104/RT) and D2=1.06×108·exp (-1.31×105/RT), respectively. The grain growth equations of two experimental steels at different soaking time from 15 to 120 min with the soaking temperature of 840°C are as follows: the equation of No.1 steel is D1=4.83×10-2·t0.72 while that of No.2 steel is D2=1.25·t0.18.

Surface Decarburization Behavior of Spring Steel 60Si2MnA under AC1 Temperature and in Temperature Range AC3-G

Surface decarburization of spring steel 60Si2MnA heated under AC1 temperature and in temperature range of AC3-G was investigated. The results show that under the AC1 temperature, pearlite spheroidization and surface decarburization were carried out simultaneously and had a mutual promotion relationship. In the temperature range of AC3-G, decarburized layer consisted of complete and partial decarburization, and the complete decarburized depth increased but the partial decarburized depth just a small change with the increase of heating time. Besides, when heating temperature was 790 °C, functional relation between the total decarburized depth, the complete decarburized depth and the heating time follows the formula of and , respectively.

Network Carbide Inheritance during Heat Treatment Process of Large Shield Machine Bearing Steel GCr15SiMn

Network carbide inheritance during heat treatment process of large shield machine bearing steel GCr15SiMn was investigated by heat treatment experiments and quantitative metallographic. Samples with the proeutectoid cementite network thickness in the range of 0.19~0.54 μm were obtained by changing austenitizing temperature and soaking time of pearlite transformation. The results show that the network in hot rolled bar can be improved when the pre-heat treatment temperature is 950 °C. When the network thickness is above 0.40 μm, the undissolved cementite networks present in microstructures after quenching and tempering. In a Chinese standard, the network grades are 1.5 and 3.0 degree when the networks thickness are 0.40 μm and 0.54 μm, respectively. The critical network thickness that can be eliminated by heat treatment is 0.29 μm.

Splitting Property Analysis of Automotive Con-Rod Steel C70S6

In order to analysis the splitting property of automotive con-rod steel C70S6, a type of con-rods using this material were manufactured on a domestic production line. Both the fracture and the cracks of the con-rods were observed by SEM, the relationship between the cracks and the grain orientation were analyzed by EBSD. The results show that the fracture of the steel is cleavage fracture and most cleavage planes are {001} , the crack initiation is mainly ferrite or inclusions, and the crack propagation race is along the {001} planes, most microcracks in the vicinity of the main crack induced by the notch can coalesce with it and change its propagation direction, this is the probable formation mechanism of the step-like fracture.

Research on Fatigue Fracture Defect of 23CrNi3Mo Drill Steel

In order to analyze the key factor to affect fatigue life of drill tools, fracture morphology and microstructure of defect samples made of 23CrNi3Mo with different types was observed by optical microscope and scanning electron microscope(SEM), and micro-hardness was tested, then failure reason was analyzed. The results show that the plastic deformation features of defect sample with high fatigue life is obvious during fatigue cracks generation and expanding, which has good ability to resist fatigue crack propagation, while impacter drill defect sample appears brittle fracture features. The surface of Atlas and impacter drill both have carburized layer with high hardness. The wide transitive layer of Atlas drill renders the strength of carburized layer and the roughness of matrix well matched and has well hardness gradient, while the impacter doesn’t. Thus the hardness gradient and the matching of strength and toughness affect the drill fatigue life heavily, and carburization and heat treatment process are very important.

Influence of Cooling Rate and Chemical Composition on Phase Transformation and Hardness of C70S6 Steel

The influence of cooling rate and chemical composition on phase transformation and hardness of C70S6 steel were studied by Gleeble-3800 thermal simulation testing machine and box type electric furnace. The results showed that when the cooling rate was between 0.3 and 5 °C/s, the transformation products of two experimental steels were mainly composed of ferrites, pearlite and sorbite. The pearlite content gradually decreased with the cooling rate increasing, while the sorbite content increased and the ferrite content changed little. Both the ferrite and pearlite transformation starting temperature and ending temperature decreased with the cooling rate increasing. Besides, the hardness increased with the cooling rate. At the same cooling rate, the phase transformation temperature increased slightly with the carbon equivalent decreasing, and the pearlite content increased, while the hardness decreased. The hardness of C70S6 steel was reduced by cooling rate decreasing. However, it was difficult to realize the method of decreasing the hardness by adjusting the cooling rate in the case of higher carbon equivalent. Therefore, in order to obtain an appropriate hardness, the Ceq must be controlled. And a Ceq=0.83% was recommended.

Hot Ductility of 20Cr13 Steel at High Temperature

Hot ductility of 20Cr13 steel at high temperature was investigated through tensile test. The main phases of the steel in temperature range of 600-1400 °C were calculated with thermodynamic software. The fracture morphologies and microstructure were observed by scanning electron microscopy. The steel showed good hot ductility in temperature range of 1000-1200 °C, and the area reduction was 82 % or more. In temperature range of 800-950 °C, the M23C6 precipitated at the grain boundary of austenite, and it fractured in the tensile process. Thus the micro-crack or micro-hole formed at the grain boundary of austenite, and the area reduction was 52-68 %. The ferrite precipitated at 800 or 1250 °C, and the micro-voids formed at interface of ferrite and austenite because of the discordant deformation of two phases, which was harmful to hot ductility. Moreover, quasi-cleavage fracture happened at 800 °C because of the ferrite cleavage, which further decreased hot ductility of the steel.

Effect of Alloying Elements and Heating Parameters on the Hardness of Quenched and Tempered Steel 42CrMo after Induction Hardening

The effect of alloying elements, heating rate and quenching temperature on the hardness of steel 42CrMo after induction hardening were investigated by thermal simulation tests, hardness tests, optical microscopy (OM) and scanning electronic microscopy (SEM). The steel No.2 was obtained by adding 1.49% Ni and reducing 0.32% Cr in steel No.1. Experimental results show that the hardness of steel No.1 decreases while that of steel No.2 slightly increases under the quenched temperature of 900 °C when the heating rate increases from 100 °C/s to 300 °C/s. When the heating rate is 300 °C/s, the hardness of steel No.1 increases sharply with the elevating of the quenched temperature. However, the hardness of steel No.2 increases gently with the quenched temperature increasing below 900 °C but keeps constant above 900 °C. It is interesting that the hardness of steel No.2 is obviously higher than that of steel No.1 under the same heating process. This phenomenon can be attributed to the following aspects: firstly, the Ac1 and Ac3 temperatures can largely decrease and the dissolution of carbide proceeds further; secondly, the addition of alloying element and the dissolution of carbide in the austenite can improve the hardenability of steel and thus increase the volume percentage of martensite in the quenched microstructure; in addition, the quenched microstructure of steel No.2 is finer than that of steel No.1 which undoubtedly helps the increase of hardness. In consideration the disadvantage of higher quenched temperature and heating rate, it is comprehensive to determine the quenched temperature as 900 °C and heating rate as 100 °C/s in order to increase the hardness of steel No.2.

Effect of Mo on Transformation, Microstructure and Property of Hot-Rolled DP600 Steel

In according with the transformation rules of C-Si-Mn-Cr and C-Si-Mn-Cr-Mo tested steels, rolling experiment was carried out in lab. to analyze the effect of Mo on transformation, microstructure and property of high strength hot rolled dual phase steels. The results showed that the addition of element Mo decreased start temperature of ferrite transformation and restrains pearlite transformation. There was a metastable austenite zone between ferrite and bainite transformation zone of No.2 steel with 0.35% of Mo. After controlled rolling and step cooling process, dual phase microstructures in which fine martensite islands dispersed in soft ferrite matrix were obtained by two kinds of tested steel. With the addition of Mo, yield strength and tensile strength increased by 80MPa and 60MPa respectively, meanwhile, elongation increased slightly.