Tempering cooling rate plays a significant role in the impact toughness of 2CrMoV weld metal. Three different tempering cooling rate experiments were carried out; it is found that the impact toughness of weld metal improved from 44.61 to 117.49 J as the cooling rate increased from 5 to 40 °C/h. Microstructure characterization revealed that the large blocky M–A constituents and cluster precipitation were considered to act as stress concentration sources and cleavage fracture initiators at a cooling rate of 5 °C/h. Under the cooling rate of 20 °C/h, the decrease of blocky M–A constituents as well as homogeneous distribution of precipitation induced the transition from cleavage to interfacial decohesion. The chance of crack propagation in intragranular ferrite matrix was increased, which needed to absorb more energy and improve impact toughness. When the tempering cooling rate reached at 40 °C/h, the cracks mainly propagated in the ferrite matrix; meanwhile, fine and homogeneous distribution of precipitation greatly inhibited crack propagation and led to higher impact toughness.

Process parameters (laser power and scanning speed) for H13 steel specimens produced by selective laser melting (SLM) are optimized, and microstructural characteristics and mechanical properties are investigated. The optimum process parameters are a laser power of 170 W and a scanning speed of 400 mm/s according to the maximum relative density of 99.2%. The microstructure consists of cellular grains and columnar crystal, which are composed of lath martensite and retained austenite, and there are no carbides or other second-phase particles present. The size of cellular grains is 1 μm. Compared with the common processed (forged and heat-treated) H13, SLM H13 has similar microhardness (561 HV) and tensile strength (1909 MPa) values. However, the elongation (12.4%) is a factor of ∼3 times higher and the impact energy (14.4 J) of the SLM specimen is somewhat lower. The relationship between the microstructure and mechanical properties is discussed. Fine grains and no second-phase precipitation determine the strength and plasticity of SLM samples.

A novel stainless steel porous twisted wire material (PTWM) is made of twisted short wires by compaction followed by vacuum high-temperature solid-phase sintering. The twisted short wires are fabricated by using a self-developed rotary multicutter tool to cut stainless steel wire ropes. The PTWMs with 46–70% porosities have been investigated in terms of porous structures and Charpy impact behavior. The PTWMs with spatial composite intertexture structures exhibit interconnected open-pore microstructures with a variety of shapes and sizes. The pore size distributions became convergent with decreasing porosities. The span of pore distribution of the PTWM with a diameter of 90 μm was half than that of the PTWM with a diameter of 160 μm under 65–66% porosity. The impact toughness of the former is 2.6 times than that of the latter. By increasing the porosity from 46 to 70%, the impact toughness decreases from 17.9 to 9.1 J/cm2. Macroscopically integral failure-morphologies of the PTWMs present mixed ductile–brittle failure mechanisms, but microscopic impact deformation and failure mechanisms mainly show the ductile failure and fracture of pore skeletons. The PTWMs demonstrate complex energy absorption mechanisms.

Dissimilar joints of advanced 9Cr/CrMoV have been successfully welded by narrow gap submerged arc welding using multi-layer and multi-pass techniques. The objective of our study is to establish the correlation between impact toughness and microstructural characteristics of the welded joints. Impact toughness tests were conducted in a wide range of temperature from −60 °C to 80 °C for different regions in the dissimilar joints. The fracture appearance transition temperature of base metal of 9Cr, CrMoV and weld metal were tested as 23 °C, −9 °C and −2 °C respectively, which all satisfied the service requirement. Optical microscope and scanning electron microscope revealed that weld metal and base metal of CrMoV comprised martensite and bainite while 9Cr was composed of lath martensite. The low toughness in the latter region arose from large grains with excessive carbide precipitates. Nonuniform microstructure in the heat-affected zone of 9Cr side caused different crack propagation paths and subsequently led to large variations of absorbed energy. When crack propagates along carbon-enriched zone in heat affected zone, the absorbed energy was 48 J. With crack deviating far from carbon-enriched zone, the absorbed energy increased to 147 J. Examination on fracture surfaces revealed the typical brittle fracture appearance in 9Cr and inter-granular fracture mode in heat-affected zone of 9Cr side when crack propagated along carbon-enriched zone.

This paper details an investigation into the effect of martensite–austenite (M–A) constituent on impact toughness of intercritical heat-affected zone (IC HAZ) in Q690 steel, a low carbon bainitic steel. Large samples with uniform microstructure similar to that of the actual IC HAZ were achieved through combinations of heat treatments. The samples were heated to temperature between 750 and 830 °C for 30 min and then quenched in water. After these heat treatments, hard martensite islands distribute along the boundaries of soft matrix, and the size of martensite islands increases with heating temperature. The microstructure is quite similar to that of the IC HAZ. Using these large samples, the impact toughness could be measured conveniently. It was found that the embrittlement of the IC HAZ depends on the deformability and size of the M–A constituent. When the size of the M–A constituent whose deformability is low exceeds a critical value (2.0 μm), the embrittlement of the IC HAZ occurs.

Various heat treatments and thermal simulation with different austenitizing temperatures and austenite deformation were applied on a bearing steel to obtain various austenitic state. The effect of austenitic state on microstructure of martensite/bainite (M/B) dual phase steel and its mechanical property has been investigated via microstructure observation and kinetic analysis. The results show that the M/B steels austenitized at 900 and 950 °C have better comprehensive performance compared with the steels austenitized at 850 and 1050 °C. The refined microstructure can be obtained after deformation, and the heavy deformation and low deformation temperature are useful for refining the microstructure. The bainite lath is longer and has well-directional arrangement at high austenitizing temperature with the same deformation. Furthermore, austenite deformation can improve the nucleation ratio, reduce incubation process, and affect the kinetics of bainite transformation significantly.

Embrittlement of duplex stainless steels after aging below 500 °C is a well documentedphenomenon, usually attributed to the unmixing of the ferritic Fe-Cr solid solution,either by spinodal decomposition or by nucleation and growth of Cr-rich α′ phase,depending on aging temperature. Additional precipitation taking place during aging, likeNiSiMo-rich G phase or Cu-ε, are also known to participate to embrittlement, making thecomposition dependence of embrittlement kinetics even more intricate. In this study, longterm aging treatments, up to 2 years at intermediate temperature (250–400 °C), have beenperformed for various duplex stainless steels grades containing different alloyingcontents in Cr, Mo, Si, Ni and N. Impact toughness has been measured after each agingcondition, and an apparent activation energy could be deduced for the various grades. Thisquantity is a good indicator of embrittlement resistance. Fine microstructuralcharacterizations have been performed in the ferrite: composition before aging wasdetermined by EPMA, and precipitation state before and after aging was observed on TEM.The influence of alloying elements on the embrittlement phenomenon is analyzed on thebasis of these results and of driving force calculations performed onThermo-Calc®.

Les aciers inoxydables ferritiques connaissent un développement régulier depuis de nombreuses années, en partie grâce aux variations importantes du coût des éléments d’alliage et notamment du nickel qui impactent fortement le prix des nuances austénitiques classiques. Toutefois, il ne faut pas négliger les avantages techniques que présentent les nuances ferritiques. Cet article présente une série d’exemples illustrant les performances des aciers inoxydables ferritiques dans des domaines d’applications variés et souligne la large gamme de propriétés qui peuvent être obtenues.