Materials Science Forum Vols. 571-572

Abstract: The early years of neutron stress measurements are recounted using published documents and input from workers in the field. The circumstances and motivations of the early workers in the field are discussed, and some general conclusions are drawn. The first known reference is from the US National Bureau of Standards (NBS), now the National Institutes for Science and Technology (NIST), in 1976. In Europe, in the 1970s, materials scientists and engineers were encouraged to use neutrons to study applied problems after the ILL was commissioned, and this outreach effort was productive. The idea was also discussed in Australia at this time. Actual depth-probing measurements of stress began in 1979 at Missouri and Karlsruhe, then Harwell in 1980. The 1980s saw dramatic growth in the number and kinds of measurements, including initial pulsed source studies at IPNS and commercial work at Harwell and Chalk River. Two meetings are particularly significant: the 28th Sagamore Army Materials Research Conference on Residual Stress and Stress Relaxation, held in July, 1981, in Lake Placid, New York, and the NATO Advanced Research Workshop on Measurement of Residual and Applied Stress Using Neutron Diffraction, held in March, 1991, in Oxford. At the Sagamore Conference, the first workers to make successful measurements met. At the NATO Workshop, the neutron stress measurement community essentially came into existence.

Abstract: The compressive stress distribution below the specimen surface of a nanocrystalline medium carbon steel was investigated nondestructively by using high-energy X-rays from a synchrotron radiation source, SPring-8 (Super Photon ring-8 GeV) in the Japan Synchrotron Radiation Research Institute. A medium carbon steel plate was shot-peened with fine cast iron particles of the size of 50 μm. By using the monochromatic X-ray beam with three energy levels of 10, 30 and 72 keV, the stress values at the arbitrary depth were measured by the constant penetration depth method. The stress was calculated from the slope of the sin2ψ diagram. Measured stress corresponds to the weighted average associated with the attenuation of the X-rays in the material. The real stress distribution was estimated by using the optimization technique. The stress distribution was assumed by the third order polynomial in the near surface layer and the second order polynomial. The coefficients of the polynomials were determined by the conjugate gradient iteration. The predicted stress distribution agreed well with that measured by the conventional surface removal method.

Abstract: Seamless tubes are used for many applications, e.g. in heating, transport gases and fluids, evaporators as well as medical use and as intermediate products for hydroforming and various mechanical applications, where the final dimensions normally are given by some cold drawing steps. The first process step – piercing of the billet, for example by extrusion or 3-roll-milling - typically results in ovality and eccentricity in the tube causing non-symmetric material flow during the cold drawing process, i.e. inhomogeneous deformation. Because of this non-axisymmetric deformation and of deviations over tube length caused by moving tools, this process step generates residual stresses. To understand the interconnections between the geometrical changes in the tubes and the residual stresses, the residual strains in a copper tube had been measured by neutron diffraction.

Abstract: Experimental investigation of residual stresses after heat treatment and grinding processes in the production of ball bearing rings has been carried out. The residual stresses were measured by X-ray diffraction method utilizing chromium radiation, which has an average penetration depth of 5 μm incident on 100Cr6 (AISI-E52100) ball bearing steel. The process parameters of heat treatment and grinding processes were varied so as to represent the extreme values that can be applied in the respective processes. Hardness and percent retained austenite limit the heat treatment process parameters; while roundness, surface roughness and form the grinding process. Tensile surface residual stresses on the raceway of ball bearing rings changes to compression after grinding in both circumferential and axial directions. In grinding relatively higher compressive stresses were measured in axial direction compared to the circumferential direction. This experimental investigation also showed that the influence of heat treatment process parameters on the magnitude and distribution of residual stresses survived even after grinding process; i.e. heat treatment and grinding processes cannot be evaluated independently in process design for favourable residual stresses.

Abstract: A full review for technological processes used today, in dental prostheses industry for titanium casting, both in Europe and in United States, reveal that, there is no equipment available on the market able to cast small titanium parts in secondary inert vacuum by induction melting and centrifugal casting. All these advanced processes together contribute to an original concept of the Denticast System. The computer controlled cast sequence is another important factor contributing to a robust system, able to reproduce the achieved technological quality in a reliable manner. Mechanical testing (tensile and hardness) shows that the different alloys used in Denticast project and cast using the prototype, are in most of the results superior to those obtained with two different commercial systems (pressure-vacuum and centrifugal systems). The samples are usable for dental prosthesis without any risk for the patient health.

Abstract: Mechanical and thermal treatments during the manufacturing process inevitably cause the accumulation of residual stresses in parts consisting of materials with complex microstructures. Neutron diffraction is particularly well suited to determine residual stress distributions within the bulk of the component. Due to the nature of a diffraction experiment an inextricable mixture of type I and II residual stresses is measured. The accumulation of type II stresses (microstresses) is strongly related to the microstructure. The impact of changes in the microstructure on neutron diffraction experiments has been investigated on Inconel 718 (IN718) samples.

Abstract: The most critical stage in the heat treatment of high strength aluminium alloys is the rapid cooling necessary to form a supersaturated solid solution. During cold water quenching of thick sections, the thermal gradients are sufficient to cause inhomogeneous plastic deformation which in turn leads to the development of large residual stresses. Two 215 mm thick rectilinear forgings made from 7075 and 7010 were heat treated, and the through thickness residual stresses measured by neutron diffraction and deep hole drilling. The distribution of residual stresses was found to be similar for both alloys varying from highly triaxial and tensile in the core to a state of biaxial compression in the surface. The 7010 forging exhibited significantly larger tensile stresses in the core. 7075 is a much more quench sensitive alloy when compared to 7010. This results in loss of supersaturation by second phase precipitation during quenching in the core of the 7075 forging.

Abstract: Cold-drawing is employed to fabricate wires and rods, which are mainly used as structural reinforcements in construction as well as in the tyre industry. As a consequence of processing, a residual stress profile is developed. In this paper, residual stress profiles are measured by neutron diffraction in cold-drawn pearlitic steel rods subjected to different deformations (true strain from 0.3 to 1.7). The results show that the residual stress profile produced by cold-drawing is similar in all the samples, irrespective of the degree of deformation.