V. Schulze

Residual stress relaxation in an AISI 4140 steel due to quasistatic and cyclic loading at higher temperatures

Abstract Residual stresses can be relaxed by supplying sufficiently high amounts of thermal and/or mechanical energy, which converts the residual elastic strains to microplastic strains. In order to better understand this relaxation behavior, shot peening induced residual stresses in normalized condition and in quenched and tempered condition of the steel AISI 4140 (German grade 42 CrMo 4) were investigated in annealing experiments, quasistatic loading experiments and bending fatigue experiments at 25, 250 and 400°C. The residual stress relaxation during alternating bending occurs in different regimes. First, thermal relaxation reduces the residual stresses during specimen heating. The relaxation during the first cycle can be discussed on the basis of the effects due to quasistatic loading, if the inhomogeneous distribution of the loading stress is taken into account. Differences in the behavior after the two heat treatments result from the Bauschinger-effect and effects of dynamic strain ageing. Owing to cyclic creep effects, the interval between the first cycle (N=1) and the number of cycles to crack initiation Ni is characterized by residual stresses which decrease linearly with the logarithm of N. Finally for N>Ni the reduction of residual stresses with the logarithm of N is stronger than linear.

Effects of warm peening on fatigue life and relaxation behaviour of residual stresses in AISI 4140 steel

A new device has been built which allows shot peening in an air blast machine at elevated temperatures. The effects of conventional shot peening and peening at elevated temperatures on the characteristics of regions close to the surface, on the stability of residual stresses and half widths of X-ray interference lines and on the fatigue strength are presented for a quenched and tempered AISI 4140 steel (German grade 42CrMo4). The alternating bending strength is increased by warm peening compared with conventional shot peening. Additional investigations of samples conventionally peened and then annealed confirm that these effects are due to the stability of the dislocation structure, which is highly affected by strain ageing effects. This causes an additional benefit owing to higher stability of the residual stresses induced.

Depth profiles of macro residual stresses in thin shot peened steel plates determined by X-ray and neutron diffraction

Abstract The aim of the present work is to clarify, whether the near surface residual stresses induced by surface treatments like shot peening are balanced by low-level tensile residual stresses all over the core of the specimen or by relatively high tensile residual stresses in a thin layer adjacent to the surface layer. Therefore X-ray and neutron diffraction analyses were performed and evaluated.

INVESTIGATION OF SIZE-EFFECTS IN MACHINING WITH GEOMETRICALLY DEFINED CUTTING EDGES

The miniaturization of cutting processes shows process specific size-effects like the exponential increase of the specific cutting force k c with decreasing depth of cut h. Experiments were carried out in an orthogonal turning process. The influence of different process parameters on the results was investigated separately to identify process specific size-effects. Two materials were studied: a normalized steel AISI 1045 and an annealed AISI O2. To complement the experiments, parameter variations were performed in two-dimensional, thermo-mechanically coupled finite element simulations using a rate-dependent material model and analyzed by similarity mechanics. The influence of rounded cutting-edges on the chip formation process and the plastic deformation of the generated surface were determined numerically. The complex physical effects in micro-cutting were analyzed successfully by finite element simulations and compared to experiments.

Classification of microstructural changes in laser hardened steel surfaces

Three different steels with C contents of ∼0.2 wt.% and different contents of chromium, molybdenum and vanadium were similarly laser hardened after different preceding heat treatments. The laser-affected zones beneath the surface were characterised by microhardness, dislocation density, carbide type, size and distribution as well as grain size of the austenite. By a systematic study of the changes in microstructure of these laser-affected zones, the effective hardening mechanisms were separated and their influence on the material strength or the microhardness was classified.

Modeling the process-induced modifications of the microstructure of work piece surface zones in cutting processes

Cutting processes lead to mechanical and thermal loading of tool and work piece. This loading entails a direct influence of the cutting process on the surface layers of the manufactured work pieces. As a result, residual stresses and modifications of the micro-structure like white layers can occur in surface-near zones of the work piece. This paper presents the development of a FE-simulation model to predict phase transformations due to cutting processes. Therefore a 2D-FE-cutting simulation including a dynamic re-meshing is combined with a simulation routine to describe phase transformations that was primarily developed to simulate laser hardening. This paper illustrates the implemented mechanisms to determine phase transformations considering short time austenization and shows first experimental results revealing the influence of process parameters on the surfaces microstructure.

Influence of alloying elements on the strain rate and temperature dependence of the flow stress of steels

After a short introduction to the theoretical background of thermally activated glide of dislocations, a constitutive model is presented, which describes the temperature and strain-rate dependence of the flow stress. The properties of this constitutive equation were estimated for several plain carbon steels in normalized conditions, for quenched and tempered low-alloy steels, as well as for some high-strength low-alloy (HSLA) steels based on the temperature dependence and strain-rate sensitivity of the flow stress at temperatures 81 K≤T≤398 K and strain rates 5 · 10−5 s−1≤ε≤1 · 10−2 s−1. The constitutive equation enables the extrapolation of flow-stress data to higher strain rates (ε≲10+4 s−1), which are in good agreement with the results obtained from high strain-rate deformation tests. The influence of solute-alloying elements on the thermal stress, the activation enthalpy, and the constitutive parameters will be discussed.

INFLUENCE OF FRICTION AND PROCESS PARAMETERS ON THE SPECIFIC CUTTING FORCE AND SURFACE CHARACTERISTICS IN MICRO CUTTING

For the production of small quantities of micro devices, machining is a low cost alternative to lithographic processing techniques. However, machining shows process specific size-effects upon miniaturization to the micrometer regime. Hence, the orthogonal turning process is chosen to study the influence of process parameters like uncut chip thickness h, cutting velocity vc and cutting edge radius rβ on the cutting force and the surface plastification by two-dimensional, thermo-mechanically coupled finite element simulations. A rate-dependent plasticity law is used for investigation of a normalized medium carbon steel (AISI 1045). Furthermore, the characteristics of the influences of the different parameters are analyzed mathematically by similarity mechanics. In particular, the frictional effects on the cutting process are studied in detail using a friction coefficient μ based on experimental results, and the influences of the process parameters on the cutting force and the plastic deformation of the surface layer are determined numerically. These results are compared with experimental measurements. The specific cutting forces are analyzed and discussed in detail. Size-effects observed experimentally are also found by numerical simulations.

Development of a Simulation Model to Investigate Tool Wear in Ti-6Al-4V Alloy Machining

Titanium alloys like Ti‑6Al‑4V have a low density, a very high strength and are highly resistant to corrosion. However, the positive qualities in combination with the low heat conductivity have disadvantageous effects on mechanical machining and on cutting in particular. Ti‑6Al‑4V forms segmented chips for the whole range of cutting velocities which influences tool wear. Thus, optimization of the manufacturing process is difficult. To obtain this goal the chip segmentation process and the tool wear are studied numerically in this article. Therefore, a FEM model was developed which calculates the wear rates depending on state variables from the cutting simulation, using an empirical tool wear model. The segmentation leads to mechanical and thermal load variations, which are taken into consideration during the tool wear simulations. In order to evaluate the simulation results, they are compared with experimentally obtained results for different process parameters.

Mechanical Properties of Compound-Extruded Aluminium-Matrix Profiles under Quasi-Static Loading Conditions

Compound-extruded unidirectionally reinforced lightweight profiles are a novel class of materials for the realisation of load-bearing structures. They may be fabricated in a flexible and rapid near-net-shape process. The authors present investigations of the reinforcing effect of wires in compound-extruded aluminum profiles under quasi-static tension and compression. In particular, the compounds were characterized by metallographic examinations focusing on the fracture morphology. Furthermore, specimens subject to compression tests were examined using micro computer tomography (µ-CT) and light microscopy (LM). It is shown, that the mechanical properties of wire-reinforced profiles are improved under both positive and negative quasi-static loads in comparison to non-reinforced profiles.