Development and Microstructural Improvement of Spin Cast High-Speed Steel Rolls

A detailed microstructural analysis was conducted on a series of radial shell samples extracted from commercially produced centrifugally spin casted high-speed steel (HSS) work rolls for finishing hot strip mills (HSM). The systematic microstructural analysis was coupled with a numerical and experimental investigation to improve the life of HSS rolls. An integrated computational-experimental approach was developed to optimize the response of the HSS roll material that permitted the enhancement of the microstructure and properties of the HSS roll shell layer. Local continuous microstructural transformations through the thickness of the shell: carbide formation, precipitation, dissolution sequence and phase changes, were studied in great details. The analyses were conducted with the aid of advanced metallographic and experimental methods, finite-element (FE) analysis, and using commercial software systems to conduct thermodynamic-kinetics predictions. In order to analyze a response of the HSS roll to the hardening heat treatment (HT) and to control stress-strain evolution, a 3-D FE model was developed of the composite structure of the roll. The multilayered model considers nonlinear material properties of each individual layer as a function of temperature, based on measured chemical composition gradients through the HSS shell. Transient coupled thermal-stress analysis was performed, using actual measured surface temperatures as boundary conditions (BC) for the FE model. The allowable thermal stress-strain levels were established and compared with a) thermodynamically predicted high temperature mechanical properties and b) room temperature test results of the shear strengths for the shell, bonding and core. In addition, sub-structuring and image-based processing techniques were implemented to aid in the development of a meso-scale FE model to simulate the local response of a given microstructural constituents and matrix under particular thermal conditions. The fundamental interpretation of multilayered structure and multi-scale approach help to understand the kinetics phenomena associated with continuous local microstructural transformations due to nonlinear heat transfer. The results from the microstructural observations were in good agreement with the numerical predictions. The major impact of this work clearly indicated that a refined as-cast structure prior to the heat treatment promoted an increased precipitation of carbides during final hardening, which greatly improved strength and performance. A non-conventional HT was defined and implemented in order to provide an additional degree of microstructural pre-conditioning, which homogenized the matrix throughout the HSS shell. The new HT defined the austenitization temperatures and times to modify the morphology of brittle interdendritic eutectic carbide networks and, hence, facilitating the kinetics of dissolution of these carbides. This behavior caused an increase in the solute content of the matrix. As a result, the matrix hardness and strength were increased during subsequent hardening HT in comparison to the conventional HT routes used for as-cast HSS rolls. Reports about rolls with the new material that have been placed in service indicate that the rolls last 50-70% longer

Failure prediction of spot welded boron steel

A methodology of material characterisation and finite element model discretisation is presented for spot welded boron steel sheets, with the aim of predicting failure during quasi-static loading. The predicted load-displacement curves from the Finite Element model are compared with experimentally measured curves for lap-shear and cross-tension weld destructive geometries, and serve as model validation. During spot welding, the weld and surrounding material are exposed to a wide range of temperatures, from the melting point at the weld centre to room temperature in the base material. As a consequence, the weld exhibits varying microstructures with corresponding varying material properties which have a profound influence on its load bearing capacity and failure strength as a whole. In addition, boron steel spot welds exhibit unique hardness profiles, with high hardness values in the nugget and outlying base material, and a sudden drop in the area between these regions. This sudden decrease in material properties leads to further difficulties in modelling the failure of boron steel welds. The weld process inherently produces localised residual strains which also need to be accounted for in the model simulation, together with significant plastic strain redistributions resulting from the mechanical loading of the spot weld to its ultimate failure. The initial residual strains were measured in weld samples using neutron diffraction and were subsequently input into the FEA models. This thesis aims to quantify the varying material constitutive behaviour throughout the weld, required for the failure prediction. In particular, the following constitutive properties were extracted: the stress-strain response of certain weld regions, failure loci consisting of fracture strain versus stress state for the corresponding regions, and the residual stress distribution through the weld. Due to the small size of the weld, cutting test specimens directly from the weld is unachievable. To overcome this problem, specific weld and heat affected zone micro-structures were recreated onto practical tensile specimens through use of a Gleeble thermo-mechanical physical simulator. These specimens were subjected to the same thermal histories as specific points in the actual weld. From these tensile specimens, stress-strain curves relating to specific weld regions could be obtained. In a similar fashion, three additional destructive specimen sets were created to obtain failure loci. These failure loci give fracture strain as a function of stress state: specifically shear, uniaxial and plane-strain states. Due to the practical limitations in the accuracy of the Gleeble technique, deviations from the target microstructures were expected in the Gleeble samples. To gauge the extent of these deviations, a method of extracting reference material properties directly from the weld was required. Instrumented indentation offers such a solution, where the load and displacement of the indenter are measured and run through an algorithm to calculate the yield strength of the indented locations. These yield strengths are then compared with the yield from the Gleeble stress-strain curves to gauge the accuracy with which the weld microstructures were recreated. This technique serves to quantify the deviation of the Gleeble microstructures from the target material microstructures. It is common practice to discretise the weld into a small number of bulk regions during the design process, with material constitutive behaviour assigned to these discretised parts. In the new methodology, the extracted material constitutive behaviour is modelled as a continuously varying function of the distance from the weld centre. By performing appropriate interpolation, the data may be finely or roughly discretised. The data at a certain distance from the weld centre may then be assigned to the corresponding element in the finite element model. This means one may discretise the model by choosing the level of data interpolation refinement. The following results were observed in the thesis: • Residual strain distributions of boron steel spot welds, which have not been measured before, were presented. Clear correlations between hardness and residual stress distributions were seen. • A new application of instrumented indentation was attempted by verifying the accuracy of heat treated samples with respect to their target microstructures by comparing yield strengths. • The boron steel HAZ was characterised in a finer level of detail than seen in other literature works. • Through physical simulation, stress - strain and failure loci corresponding to certain HAZ areas were successfully extracted and used to model weld failure. • A new method of finite element model discretisation was presented, where material properties may be input as a relatively smooth function through the length of the model

The behaviour of advanced quenched and tempered steels during arc welding and thermal cutting

Quenched & tempered (Q&T) wear-resistant plate steels with martensitic microstructures have been in use for many years in the mining, defence, and construction industry due to their excellent mechanical properties (up to 1700 MPa of tensile strength and \u3e10% elongation to failure). These mechanical properties are achieved by utilisation of up to 0.4 wt.% Carbon (C), \u3c1.5 wt.% Manganese (Mn), microalloying with Molybdenum (Mo), Chromium (Cr), Nickle (Ni), Titanium (Ti), and sometimes Boron (B), and a combination of carefully designed thermomechanical processing schedule and post rolling heat treatment. In the last 10 years addition of \u3c1.5 % Ti was shown to provide superior wear resistance at a moderate C content. Improvement in the wear resistance was achieved via the formation of TiC hard particles embedded in the tempered martensite matrix. Moderation of the C content in Ti-alloyed steels allowed to obtain steels with relatively low hardness, high toughness, and enhanced weldability (due to the low carbon equivalent of the steel composition). A combination of moderate hardness and high toughness positively influenced the wear resistance. Fabrication of tools and equipment from the Q&T steels is carried out using conventional fusion arc welding and thermal cutting with oxy-fuel or plasma jet. The main problem, in this case, is the formation of an edge microstructure highly susceptible to cold cracking or hydrogen-induced cracking (HIC), which results in deterioration of mechanical properties, making steel unsuitable for the required application. In the case of Ti-alloyed steels, the heat input associated with thermal cutting and welding alters the TiC particle size distribution, in addition to the tempering of the martensitic microstructure, occurring in conventional Q&T steels. However, fabrication parameters may be controlled to avoid catastrophic microstructure deterioration and product failure. Generally, a type of welding process, environment, alloy composition, joint geometry, and size are the main causes of cracking after cutting and welding. Cracking susceptibility increases as the weld metal hydrogen content, material strength, and thickness increase. Cold cracking will occur if three conditions are satisfied: susceptible microstructure; type and magnitude of residual stresses; and importantly, the level of diffusible hydrogen that enters the weld pool. Cold cracking can be avoided through the selection of controlled heat input (depends upon current, voltage, and travel speed of welding) and preheating temperature

Microstructural evolution in Grade 91(9CR-1MoVNb) power plant steels

The aim of this research project was to gam a complete, quantified, understanding of microstructural changes in high Cr ferritic-martensitic power plant steels, as a function of preservice heat treatment, stress, time and temperature. The creep strength, which is the main design criteria for this class of alloys, depends on the stability of the microstructure, which consists of tempered martensite and a fine dispersion of carbide precipitates. An understanding of the changes of these two features forms an essential process towards the creation of a physically or microstructural-based model, which may improve the current approaches towards the prediction of remanent operational lifetime of these materials in service in conventional fossil-fired power plant... cont'd