Internal Stresses and their Sources in BCC and FCC Steels

The present work summarizes and presents separate results obtained by the authors when investigating mesoscopic and microscopic internal stresses formed under the conditions of thermal and mechanical treatment of martensitic, pearlitic and austenitic steels. Internal stresses were investigated using the method based on the analysis of bend extinction contours. The results obtained on industrial steels were presented. The sources were described and examples of internal stresses induced by these sources were given. The nature of bending-torsion of the crystal lattice depending on the averaging volume was determined. It has been shown that in martensitic steels along with the increase in the averaging volume (carbide particle → separate martensitic lath → martensite packet→ martensitic plate → grain) the amplitude of bending-torsion of the crystal lattice decreases. The nature of distortions also changes. At large amplitudes and low volumes of averaging they are completely or partly elastic, at large volumes of averaging they are completely plastic. Thereby, distortions are fully driven by the excess dislocation density.

Effect of Energy Input of Welding Thermal Cycles on the Cryogenic Toughness of the CGHAZ in Steel 9Ni

The effect of energy input of welding thermal cycles with different t_(8/5) on the cryogenic toughness of the coarse-grained heat-affected zones(CGHAZs) in steel 9Ni was invested. The CGHAZ specimens for different energy input welding thermal cycles were prepared by GLEEBLE3500 thermal simulative experimental machine. The fracture morphology of welding thermal cycle specimens with different t_(8/5) was observed by using SEM. The microstructure, prior austenite grain and the appearance of packet in HAZ were observed by using OM. The block width was measured by using EBSD. The cryogenic toughness of the welding thermal cycle specimens becomes better with increasing t_(8/5). With increasing t_(8/5)，the appearance of granular bainite makes the space of martensite diminish, which refines the packet and block sizes. The refinement of martensite packet is the reason of the improvement of cryogenic toughness in steel 9Ni.

The Influence of Austenitization Temperature on the Mechanical Properties of a Prehardened Mould Steel

The many practical difficulties and longer lead time with heat treatment of mould steels after machining have led to increased demand for steels in prehardened condition, typically ~ 40HRC. At this hardness the steel possesses an optimal combination of high strength and machinability. The steels used for moulds require a wide range of demanding properties, among which high enough strength and toughness are the primary necessities in order to resist any deformation and dimensional change in mould during use. Uddeholm Impax HH which resembles the modified AISI P20 has been widely used for moulding of plastics and die-casting of low melting temperature metals. The common hardening process for Impax HH is conventional quenching and tempering. Hence, investigating the effect of hardening parameters on the required properties upon this steel grade is beneficial in improving it for better performance as a prehardened mould steel. In the present work, the effect of changes in austenitization temperature and consequently the prior austenite grain and martensite packet sizes on the tensile properties and impact toughness of Uddeholm Impax HH at the hardness of ~40HRC is studied. The results have shown reduction in impact toughness but no considerable change in yield and ultimate tensile strength upon increasing the austenitization temperature.

Creep Characteristics and Micro-Defects of Main Steam Pipe Steel at High Temperature

The initiation and growth of micro-defects such as micro cracks and voids usually causes the failure of long term operated structural components at high temperature. In this study, the creep characteristics and void nucleation and growth characteristics of P92 steel which is used as main steam pipe material in power plant were investigated at several temperatures and loading conditions. The area fraction of void increased with increase of test temperature, stress, and load holding time. In case of internal defect presence, micro-voids initiated in the early stage of loading period and resulted in the increased load line displacement and crack growth rate. The microvoids were found to form along the prior austenite grain boundaries and at the martensite packet boundaries.

Estimation of the System Free Energy of the Lath Martensite Phase in High Cr Ferritic Steels

The system free energy was estimated for the martensite phase of an Fe-Cr-C ternary alloy, 12Cr2W and 12Cr2W0.5Re steels. The system free energy of the martensite phase is defined as, Gsys = G0 + Estr + Esurf , where G0 is the chemical free energy, Esurf is the interfacial energy for the boundaries in the martensite microstructure, and Estr is the elastic strain energy due to the dislocations in the martensite phase. From the experimental results on SEM/EBSD, the total interfacial energies were estimated to be 0.83J/mol for the ternary alloy and 4.8J/mol for both 12Cr2W and 12Cr2W0.5Re steels in the as-quenched state. Also, the elastic strain energies were estimated to be 7.1J/mol for the ternary alloy, 9.6J/mol for 12Cr2W steel and 9.8J/mol for 12Cr2W0.5Re steel in the as-quenched state. So, the system free energy was about 7.9J/mol for ternary alloy. On the other hand, the system free energy was about 14.4J/mol for 12Cr2W steel and 14.6J/mol for 12Cr2W0.5Re steel. So, these microstructural energies operate as a driving force for the microstructure evolution, e.g., recovery of dislocations and the coarsening of the sub-structures such as martensite-packet, -block and -lath.

Producing a Tough, High Strength Cast Steel Free of Temper Embrittlement

Temper embrittlement of a cast 3Ni-3Cr steel was produced by small amounts of either P or Mn in samples tempered at 650°C and air cooled. Auger spectroscopy examinations showed P embrittlement was produced by cosegregation of P, Ni, and Cr. Embrittlement by Mn appeared to be caused by Mn and S segregation. In coarse grained materials fractures were intergranular. In fine grained material, both intergranular and brittle transgranular fractures were observed, and Auger mapping showed impurity enrichment on transgranular fracture surfaces. Segregation and fracture on martensite packet boundaries is suggested as the origin of the transgranular fracture. Changing the tempering temperature and adding a half percent Mo solved the embrittlement problem. Water quenching was not required. A 150 mm thick casting of the new composition showed good tolerance for P and Mn, a yield strength of 500 MPa, and good toughness down to −80°C.