Materials Science and Technology

Stress relief cracking (SRC) is a potential risk during the post-weld heat treatment of fabrication welds. The present study investigates the susceptibility of Q1N (HY 80), a high strength weldable steel, to cracking in a recently developed stress relaxation test designed to simulate stress relief conditions. It has been found in previous investigations of the behaviour of 2.25Cr-1Mo and Mn-Mo-Ni steels in this test that the coarse grained heat affected zone (HAZ) is the most susceptible microstructure. In the present work, a comparison has been made of cracking behaviour in the HAZ of a metal inert gas butt weld and also in plate material in which the coarse grained HAZ microstructure has been simulated by application of an appropriate thermal cycle. The micromechanisms of cracking have been examined in both conditions and the prediction of SRC behaviour in service weldments from the observed behaviour of simulated HAZs is evaluated. It was found that cracking occurred under applied stresses of a magnitude which might be typical of the residual stress levels in service. Two fracture modes (observed by previous workers in other susceptible steels) were present in both the ‘welded’ and simulated conditions. The severity of cracking was also found to be similar in both conditions when plotted as a function of available susceptible material. It is concluded that observations of simulated HAZ behaviour can provide a realistic estimation of service behaviour.

The AA 5083 alloy was deformed in torsion in the ranges 300–500°C and 0.1–1 S⁻¹ to a strain of 5. The flow stress as a function of strain rose to a maximum value then gradually decreased towards a steady state and had a dependence on temperature and strain rate of the traditional form with an activation energy higher than that for pure Al. Optical microstructures of specimens quenched after working ranged from elongated grains at 300°C to recrystallized grains at 500°C. Transmission electron microscope subgrain structures from all conditions of working were more recovered at higher temperatures and lower strain rates. The subgrain diameters are smaller than those in commercial Al mainly as a result of the increased density of particles and this is in agreement with observations in extrusions. Since the degree of recovery is not substantially less than in commercial Al, one is led to confirm the theory that the dynamic recrystallization is caused by particles of <0.6 μm.

The mechanisms of, and susceptibility to, stress relief cracking (SRC) were studied in A 508 class 2 and A 533 B pressure vessel steels by means of fixed displacement, load relaxation tests on notched specimens in pure bending at 823–923 K in vacuum. The specimens had previously been given heat affected zone simulation treatments in the range 1323–1573 K. The mechanisms of SRC were found to be the same as reported separately for A 508 2 steel, which was found to be generally more susceptible to SRC than A 533 B steel because of the faster formation of microcracks and coalescence of microcracks into a macrocrack. This can be rationalized in terms of the higher creep strength in A 508 2 steel owing to the presence of chromium, and the higher concentration of intergranular sulphur or reprecipitated sulphides owing to the lower manganese content of A 508 2 steel, which results in less stable sulphides. These factors are discussed in the framework of a recently proposed model for the brittle mode of SRC.

Conventional theory on the dissolution of a spherical particle of a second phase in an alloy is shown to be inappropriate when considering carbide species. In particular, the assumptions of an error function solute concentration profile and a constant equilibrium solute concentration level at the interface cannot be made. A model for computer simulated dissolution is therefore proposed which is based on the diffusion of solute away from a carbide particle across discrete shell increments. The method is used to predict times for complete and partial dissolutions of M₂₃C₆ particles in Nimonic 80A (registered trade mark of Henry Wiggin and Co. Ltd). As a result, the effect of solution heat treatment of the alloy is more clearly understood and practical improvements are recommended.

The react diffusion under pressures of 0–3 GPa has been investigated in the Al-Ag system at temperatures of 729–873 K. The only phase appearing in the diffusion zone under pressures of 0–3 GPa is ς phase. The rate constants of phase growth and the interdiffusion coefficients decrease with increasing pressure. The activation volumes of interdiffusion and phase growth of ς phase are determined to be 7.9 × 10⁻⁶ and 7.0 × 10⁻⁶ m³ mol⁻¹ respectively. These values of activation volume indicate that phase growth is controlled by diffusion occurring by the monovacancy mechanism.