Effect of Heat Treatment on Microstructure and Properties of 24CrNiMo Alloy Steel Formed by Selective Laser Melting (SLM)

The 24CrNiMo alloy steel powder was used as experimental material. The microstructure and mechanical properties of as-deposited, quenched and tempered (QT) and stress-relief annealed (SR) specimens were analyzed. The X-ray diffraction (XRD) analysis showed that the specimens in the three states were mainly ferrite (α-Fe), in which the as-deposited samples had puny face-centered cubic (FCC) structure Fe (γ-Fe). The microstructure observation showed that the as-deposited specimens were made up of ferrite, granular bainite, a small amount of cementite and residual austenite. The tensile test results indicated that the tensile strength and yield strength of the as-deposited specimens were 1199 MPa and 1053 MPa respectively, and the elongation at break was 10.7%. The elongation of QT and SR specimens increased to 11.6% and 12.8%, respectively. The electron backscattered scattering detection (EBSD) analysis results showed that the small-angle grain boundary content of the as-deposited samples was 58%, and large-angle grain boundary content was 15%. After QT and SR, small-angle and large-angle grain boundaries were obtained than those in the as-deposited specimens. The high-temperature friction and wear properties and thermal fatigue performances of the QT and SR specimens were improved significantly. The QT specimens had the smallest wear and thermal fatigue crack lengths, excellent resistance to friction and wear performance and prevention of crack growth, with an ideal comprehensive properties.

Effects of angle formation between melted zone and friction direction on thermal fatigue and wear resistance of truck drum brake

The failure of grey cast iron drum brake is mainly caused by thermal fatigue crack running through the top along the axial direction, which has the characteristics of regional failure. By using the advantages of laser fabrication technology, the single models with five different angles combined with the anti-crack ring were designed. The optical microscope, scanning electron microscope and X-ray diffraction were used to characterize the microstructure and phase of the drum brake material grey cast iron and the bionic coupling unit. Both single models and double combination models were tested for thermal fatigue and wear after thermal fatigue, respectively. The effects of angle on the thermal fatigue and wear resistance of the bionic coupling models were investigated. Experimental results show that the microstructure of the unit was better than that of the base grey cast iron due to the appearance of ledeburite, martensite and residual austenite after laser treatment. Besides, thermal fatigue crack has a great influence on wear performance. The thermal fatigue crack and wear loss weight are the least in the model with anti-crack ring + 45°angle, the main crack length is the shortest. The service life of the drum brake with the double combination bionic coupling model is increased by 50% compared with that of the untreated drum brake in bench test.

Thermal Fatigue Failure Behavior of Surface/Interface of Plasma Cladding Layer

Co-based coating was prepared by plasma cladding on FV520B substrates. Microstructure of the coatings was observed by scanning electron microscope. Finite element simulation as a predictive method to research the stress distributed after thermal cycling. Thermal fatigue resistance of the coating-substrate was evaluated at temperature of 600 °C, 700 °C, 800 °C, and 900 °C. Results indicate that the surface/interface structure has excellent thermal fatigue resistance at 600 °C, and the thermal fatigue crack initiated near the interface and extended along the grain boundary. The difference of expansion coefficient of the coating and substrate is small near 600 °C, and the difference increased when the temperature climbed above 600 °C. The diffuse elements could be found near the interface after the thermal cycle, and the dislocations and precipitated phase were observed.