Investigations of Microstructures and Erosion–Corrosion Performance of Cast Boron-Bearing Stainless Steel

The microstructures and erosion–corrosion properties of boron-bearing stainless steel were researched by an erosion–corrosion tester, energy dispersive X–ray spectrometry, scanning electron microscope and X-ray diffraction analysis. The microstructures of as-cast, boron-bearing stainless steel contain M7(B,C)3, M2(B,C) borocarbides and the martensite matrix; the matrix has less chromium and more nickel than those in the M2(B,C) and M7(B,C)3. The microstructures in heat-treated, boron-bearing stainless steel consist of M7(B,C)3, M2(B,C) and M23(B,C)6 borocarbides and ferrite, and the Rockwell hardness of heat-treated, boron-bearing stainless steel is lower than that of as-cast steel. For Cr28 white cast iron and boron-bearing stainless steel, the mixing wheel with higher rotating speed leads to a higher erosion–corrosion weight loss, and as the impingement angle increases, the erosion–corrosion weight loss increases first, and then decreases. For any erosion–corrosion experiment conditions, the erosion–corrosion resistance of boron-bearing stainless steel is better than that of Cr28 white cast iron.

The Role of Chemical Composition of High-Manganese Cast Steels on Wear of Excavating Chain in Railway Shoulder Bed Ballast Cleaning Machine

The main task for a ballast bed is to transmit the sleeper pressure in a form of stress cone to the subsoil, provide proper drainage and resist the sleeper displacement. Poorly maintained ballast could severely limit the maximum speed capacity and create further problems with the structural integrity, possibly leading to a complete failure of a given rail line. To prevent the unwanted corollaries, the ballast bed has to be periodically cleaned with an appropriate machinery. In this paper the authors investigated the effect of the chemical composition on the physical properties of the ballast excavating chains made of high-manganese steels. The authors focused on the wear mechanism, work hardening ability and hardness in the cross-sections areas. A microstructure analysis was performed as well, and observations revealed divergent morphology of precipitations at the grain boundaries, which influenced the size of austenite grains. The deformation twins formed as a result of operation were noticed in the samples. Research has shown that less carbon and chromium reduces the hardness of cast steel, and it specifically affects the ability to strain hardening. The authors explained the role of adjustments in chemical composition in the operational properties of high-manganese cast steels. It has been shown in the paper that different chemical compositions affect the properties of the alloys, and this causes different types of wear. The high content of chromium increases the hardness of materials before and after plastic deformation hardening, which in the conditions of selector chains results in greater dimensional stability during wear of holes in pin joints and will be more susceptible to abrasive wear in the presence of dusts from the ballast than creep.

Microstructure and Mechanical Properties of Cast and Hot-Rolled Medium-Carbon Steels under Isothermal Heat-Treatment Conditions

In this study, the changes in the microstructure and mechanical properties during isothermal heat treatment of cast steel before and after hot deformation were investigated using medium-carbon steel with low alloy content. The microstructural characteristics of the cast and hot-rolled medium-carbon steel under isothermal heat-treatment conditions were examined using optical microscopy and scanning electron microscopy in conjunction with electron backscatter diffraction. The variation in the mechanical properties was evaluated using Rockwell hardness and tensile tests. After maintaining an austenitizing condition at 1200 °C for 30 min, an isothermal heat treatment was performed in the range 350–500 °C, followed by rapid cooling with water. Both the cast steel and hot-rolled steel did not completely transform into bainitic ferrite during isothermal heat treatment. The partially untransformed microstructure was a mixture of martensite and acicular ferrite. At 500 °C, the prior austenite phase changed to Widmanstätten ferrite and pearlite. At 450 °C, bainitic ferrite and cementite were coarsened by the coalescence of ferrite and subsequent diffusive growth. The mechanical properties increased as the isothermal heat-treatment temperature decreased, and the hardness of the cast steel was generally higher than that of the hot-rolled steel. Hardness and strength showed similar trends, and overall mechanical properties tend to decrease as the isothermal heat-treatment temperature increases, but there are slight differences depending on complex factors such as various phase fractions and grain size.

Set-up of a Generalized Dataset for Crack-Closure-Mechanisms of Cast Steel

In components, crack propagation is subjected to crack-closure-mechanisms which affect the build-up of the relevant threshold stress intensity factor range during cyclic loading. As structural parts are exposed to service loads incorporating a variety of load ratios, a significant change of the long-crack threshold value occurs, leading to a severe stress ratio dependency of crack-closure-mechanisms. Thus, an extensive number of crack propagation experiments is required to gain statistically proven fracture mechanical parameters describing the build-up of closure effects as crack growth resistance curves.The article presents a generalized dataset to assess the formation of crack-closure-mechanisms of cast steel G21Mn5+N. Numerous crack propagation experiments utilizing single edge notched bending (SENB) sample geometries are conducted, incorporating alternate to tumescent stress ratios. The statistically derived, generalized crack growth resistance curve features the impact of closure effects on the crack propagation rate in a uniform manner. To extend the dataset to arbitrary load ratios, the long-crack threshold approach according to Newman is invoked. The generalized dataset for the cast steel G21Mn5+N is validated by analytical fracture mechanical calculations for the utilized SENB-sample geometries. Incorporating a modified NASGRO equation, a sound correlation of analytical and experimental crack propagation rates is observed. Moreover, the derived master crack propagation resistance curve is implemented as a user-defined script into a numerical crack growth calculation tool and supports a local, node--based numerical crack propagation study as demonstrated for a representative SENB-sample. Concluding, the derived dataset facilitates the calculation of fatigue life of crack-affected cast steel components subjected to arbitrary stress ratios.

Design of Low-Pressure Sand Casting Process for Water-Cooled Motor Shell in Electric Vehicle

Aluminium alloy motor shell is the core part of the new energy vehicle powertrain. It has a complex structure with an average wall thickness of 4∼5mm, a side wall with 6∼7mm spiral water jacket, and the bottom of the shell inlaid with cast steel bearing bushing. Because of its complex structure characteristics, as well as higher internal quality requirements and air tightness requirements, the casting process of motor housing is very difficult. This paper introduces the structural characteristics and common casting defects of the motor shell. On this basis, the typical casting process scheme of motor shell with sand core casting technology and the application of computer simulation technology in the development of motor shell casting process are discussed and shared.