H. J. Rack

Bio: H. J. Rack is an academic researcher from Sandia National Laboratories. The author has contributed to research in topics: Maraging steel & Microstructure. The author has an hindex of 7, co-authored 10 publications receiving 305 citations.

The strength, fracture toughness, and low cycle fatigue behavior of 17-4 PH stainless steel

Abstract: The influence of microstructure on the strength, fracture toughness and low cycle fatigue behavior of 17-4 PH stainless steel has been examined. Aging hardening involves initial formation of coherent copper-rich clusters which transform to incoherent fee ∈-copper precipitates upon further aging. The changes in strength level and strain hardening rates observed during aging are consistent with previously suggested models for precipitation hardening based on differing elastic moduli. The fracture toughness and fatigue crack growth rates were shown to be a function of microstructure and environment. At equivalent strength levels overaging resulted in a higher fracture toughness than did underaging. The fatigue crack growth rates increased with increasing strength level and humidity but were not a function of toughness level. Attempts to correlate the fatigue crack growth rates with monotonie tensile properties were unsuccessful. However when final failure obeyed a critical strain criteria, the fracture toughness behavior could be reasonably described and related to preferential void nucleation and growth at δ-ferrite-matrix interfaces.

The strength and fracture toughness of 18 Ni (350) maraging steel

Abstract: The influence of microstructure on the strength and fracture toughness of 18 Ni (350) maraging steel was examined. Changes in microstructure were followed by X-ray and neutron diffraction and by optical and electron microscopy. These observations have been correlated with the fracture morphology established by scanning electron microscopy. Air cooling this alloy from the austenitizing temperature results in a dislocated martensite. During the initial stage of age hardening, molybdenum atoms tend to cluster (forming preprecipitates) and the cobalt assumes short range ordered positions. Subsequent aging results in Ni3Mo and σ-FeTi with overaging being associated with the formation of equilibrium reverted austenite and Fe2Mo. The fracture behavior is examined in terms of elementary dislocation precipitate interactions. It is suggested that the development of coplanar slip in the underaged conditions leads to its increased stress corrosion susceptibility and decreased fracture toughness. The optimum aged condition is then associated with cross-slip deformation. The fracture behavior of the overaged condition is a dynamic balance between a brittle matrix and the ductile (crack blunting) reverted austenite.

Thermal embrittlement of 18 Ni(350) maraging steel

Abstract: The embrittlement of as-solutionized 18 Ni(350) Maraging steel was monitored as a function of heat treatment variables by means of Charpy impact tests The processing parameters of interest were annealing temperatures in the range of 1900° to 2400°F, intermediate holding temperatures in the range of 1300° to 1800°F, and the quenching rate The changes in fracture mode with heat treatment were characterized by replica and scanning electron microscopy The severity of thermal embrittlement increases with decreasing cooling rate from the annealing treatment upon direct quenching to room temperature Intermediate isothermal holding, particularly at 1500° to 1600°F, further accentuates the embrittlement A large grain size is beneficial to the toughness when rapid direct quenches from the annealing range are imposed but is detrimental upon air cooling or intermediate holding The major loss in toughness may be associated with the diffusion of interstitial impurity atoms (C+N) to the austenite grain boundaries during cooling or intermediate isothermal holding below 2000°F An advanced stage of the embrittlement is characterized by the discrete precipitation of Ti(C,N) platelets on these boundaries Thermal embrittlement is accompanied by change in fracture mode from transgranular dimpled rupture to intergranular quasi-cleavage

Fabrication of Metal and Alloy Components by Additive Manufacturing: Examples of 3D Materials Science

Abstract: Objective This paper provides a brief review of relatively new additive manufacturing technologies for the fabrication of unusual and complex metal and alloy products by laser and electron beam melting. A number of process features and product microstructures are illustrated utilizing 3D optical and transmission electron microscope image compositions representing examples of 3D materials science. Methods Processing methods involving electron beam melting (EBM) and a process referred to as direct metal laser sintering (DMLS), often called selective laser melting (SLM) are described along with the use of light (optical) microscopy (OM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) to elucidate microstructural phenomena. Results Examples of EBM and SLM studies are presented in 3D image compositions. These include EBM of Ti-6Al-4V, Cu, Co-base superalloy and Inconel 625; and SLM of 17-4 PH stainless steel, Inconel 718 and Inconel 625. Conclusions 3D image compositions constituting 3D materials science provide effective visualization for directional solidification-related phenomena associated with the EBM and SLM fabrication of a range of metals and alloys, especially microstructures and microstructural architectures.

Microstructures and Properties of 17-4 PH Stainless Steel Fabricated by Selective Laser Melting

Abstract: Objective This research examines 17-4 PH stainless steel powders produced by atomization in either argon or nitrogen atmospheres (producing martensitic (α-Fe) or mostly austenitic (γ-Fe) phase powders, respectively) and correspondingly fabricated by selective laser melting (SLM) in either a nitrogen or argon atmosphere. Methods Pre-alloyed 17-4 stainless steel powders prepared by atomization in either argon or nitrogen atmospheres were fabricated by SLM. The initial powder microstructures and phase structures were examined by light (optical) microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Prototypes fabricated by SLM were similarly characterized, and in addition transmission electron microscopy (TEM) characterization was also performed. Results Martensitic powder fabricated by SLM in nitrogen gas produced a martensitic product while pre-alloyed austenitic powder produced a primarily austenitic product. In contrast, both powders produced martensitic products when fabricated by SLM in argon gas. This unusual behavior occurred because of the rapid cooling affected by nitrogen versus argon cover gas as a consequence of a 40% greater thermal conductivity of nitrogen gas versus argon gas. SLM fabricated martensitic products exhibited HRC 30 in contrast to HRC 43 when aged at 482°C for 1 hour. Austenitic products did not exhibit age-hardening. Conclusions Using an argon cover gas, SLM-fabricated products are martensitic (and magnetic) with either an austenitic or martensitic pre-alloyed 17-4 PH stainless steel powder. Using a nitrogen cover gas, the product phase is the same as the precursor powder phase (austenitic or martensitic).

Precipitation reactions and strengthening behavior in 18 wt pct nickel maraging steels

Abstract: The crystallography, structure, and composition of the strengthening precipitates in maraging steels C-250 and T-250 have been studied utilizing analytical electron microscopy and computersimulated electron diffraction patterns. The kinetics of precipitation were studied by electrical resistivity and microhardness measurements and could be described adequately by the Johnson-Mehl-Avarami equation, with precipitate nucleation occurring on dislocations and growth proceeding by a mechanism in which the dislocations serve as collector lines for solute from the matrix along which pipe diffusion occurs. The strengthening of the Co-free, higher Ti T-250 steel is caused by a refined distribution of Ni3Ti precipitates. High strength is maintained at longer times from the combined effect of a high resistance of these precipitates to coarsening and a small volume fraction of reverted austenite. In the case of the Co-containing, lower Ti C-250 steel, strengthening results from the combined presence of Ni3Ti (initially) and Fe2Mo precipitates (at longer times). Loss of strength at longer times is associated, in part, with overaging and mainly from the larger volume fraction of reverted austenite. The resistance to austenite reversion is dependent on the manner in which the relative nickel content of the martensite matrix is affected by the precipitating phases, and the difference in the reversion tendency between the two steels can be explained on this basis.

Microstructural Evolution in a 17-4 PH Stainless Steel after Aging at 400°C

Abstract: The microstructure of 17-4 PH stainless steel at various stages of heat treatment, i.e., after solution heat treatment, tempering at 580 °C, and long-term aging at 400 °C, have been studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The solution-treated specimen consists largely of martensite with a small fraction of δ-ferrite. No precipitates are present in the martensite phase, while spherical fcc-Cu particles are present in the δ-ferrite. After tempering for 4 hours at 580 °C, coherent Cu particles precipitate in the martensite phase. At this stage, the Cr concentration in the martensite phase is still uniform. After 5000 hours aging at 400 °C, the martensite spinodaly decomposes into Fe-rich α and Cr-enriched α′. In addition, fine particles of the G-phase (structure type D8a, space group Fm\(\bar 3\)m) enriched in Si, Ni, and Mn have been found in intimate contact with the Cu precipitates. Following spinodal decomposition of the martensite phase, G-phase precipitation occurs after long-term aging.

Nonlinear Solid Mechanics: Bifurcation Theory and Material Instability

Abstract: 1 Introduction 2 Elements of tensor algebra and analysis 3 Solid mechanics at finite strains 4 Isotropic nonlinear hyperelasticity 5 Solutions of simple problems in finitely deformed nonlinear elastic solids 6 Constitutive equations and anisotropic elasticity 7 Yield functions with emphasis on pressure-sensitivity 8 Elastoplastic constitutive equations 9 Moving discontinuities and boundary value problems 10 Global conditions of uniqueness and stability 11 Local conditions for uniqueness and stability 12 Bifurcation of elastic solids deformed incrementally 13 Applications of local and global uniqueness and stability criteria to non-associative elastoplasticity 14 Wave propagation, stability and bifurcation 15 Post-critical behaviour and multiple shear band formation 16 A perturbative approach to material instability