Double-step inter-critical tempering of a supermartensitic stainless steel: evolution of hardness, microstructure and elemental partitioning

The maximum allowed hardness for low carbon martensitic stainless steel components used in the oil and gas industry is 247 HV. Inter-critical tempering is an effective method for hardness control due to the production of stable reverted austenite. By conducting multiple-step tempering cycles, the austenite reversion kinetics can be accelerated and its thermal stability upon cooling can be greatly increased. In this work, supermartensitic stainless steel samples were subjected to two-step inter-critical tempering cycles. First, all samples were heat-treated at 625 °C for 2.5 h to minimize hardness through the maximization of stable austenite. Then, a second stage tempering with temperatures between 560 and 720 °C for 2.5 h was studied. The amount of stable reverted austenite at room temperature increased for second stage temperatures below 625 °C. Between 625 and 670 °C, the amount of reverted austenite notably increased at high temperature but it had limited thermal stability upon cooling. Above 670 °C, all newly reverted austenite was completely unstable during the cooling stage and partial dissolution of the stable austenite obtained after the first tempering cycle also occurred. Interestingly, hardness was mostly insensitive to the stabilization of additional austenite or to the newly formed fresh martensite158FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP2016/13466-6; 2018/21251-

Influence of hydrogen on the microstructure and fracture toughness of friction stir welded plates of API 5L X80 pipeline steel

In this work, the influence of hydrogen on the microstructure and fracture toughness of API 5L X80 high strength pipeline steel welded by friction stir welding was assessed. Samples were hydrogenated at room temperature for a duration of 10 h in a solution of 0.1 M H2SO4 + 10 mg L−1 As2O3, with an intensity current of 20 mA cm−2. Fracture toughness tests were performed at 0 °C in single-edged notched bending samples, using the Critical Crack Tip Opening Displacement (CTOD) parameter. Notches were positioned in different regions within the joint, such as the stir zone, hard zone, and base material. Hydrogen induces internal stress between bainite packets and ferrite plates within bainite packets. Besides, hydrogen acted as a reducer of the strain capacity of the three zones. The base metal had a moderate capacity to resist stable crack growth, displaying a ductile fracture mechanism. While the hard zone showed a brittle behavior with CTOD values below the acceptance limits for pipeline design (0.1–0.2 mm). The fracture toughness of the stir zone is higher than that of the base metal. Nevertheless, the stir zone displayed higher data dispersion due to its high inhomogeneity. Hence, it can also show a brittle behavior with critical CTOD values44412345823471CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ403530/2014-8; 150215/2016-9The authors are grateful to the Company TenarisConfab for sample supply and PETROBRAS for financial support. This study was financed in part by the National Council for Scientific and Technological Development, Brazil CNPq processes 403530/2014-8 and 150215/2016-9. Research supported by LNNano - Brazilian Nanotechnology National Laboratory, CNPEM/MCTI

Effect of Substrate Surface Roughness on the Tribological Properties of DLC-H Coatings on Tappet Valve

DLC-H coatings are increasingly used in automobile engine application at high temperatures, especially the reduction of friction and wear in sliding contacts. It is therefore important to determine the best substrate surface roughness on which DLC-H coatings will perform best at 200oC (extreme engine temperature). In this work, we examined DLC-H coatings on different substrate surface roughness all deposited on tappet valve by plasma enhanced chemical vapour deposition (PECVD) under similar conditions. Microstructure, structure, mechanical properties, and tribological behaviour of the coatings were examine. The film had an approximate hydrogen content of 19 %. It was observed that the hardness and elastic modulus reduced as the surface roughness increased. The coatings deposited on tappet valve of 0.3 µm substrate surface roughness possessed excellent tribological properties at 200 oC. The smoothness of the substrate aided in providing good adhesion of the coating on the substrate surface, which provided resistance to wear at 200 oC. It was revealed from the tribological behaviour and Raman analysis that the films were protected by the contact of the tribo-layer on the surface of the DLC-H coating