Cracking stress relief

If the chromium content of the steel exceeds 1.75%, the preheat is normally held until an intermediate PWHT is performed. Stress relief cracking (SRC) may occur if the sulfides are not controlled by adding cerium or zirconium, for example. Steels with more than 2.5% chromium are resistant to SRC. An empirical equation has been developed to predict the SRC of steels with less than 2.5% chromium. Cracking is predicted when aG is greater than zero where aG is defined as ... 

E. V. Barrera, M. Menyhard, D. Bika, B. Rothman, and C. J. McMahon, Quasi-static Intergranular Cracking in a Cu-Sn Alloy an Analog of Stress Relief Cracking of Steels, Met. Trans. A, to be published. 

Stress-relief cracking has been found in vessels fabricated from boron-treated carbon-molybdenum steel and in vanadium-treated steels of the type shown in Table 4-16. Laboratory tests have confirmed that molybdenum, vanadium and boron promote stress-relief cracking, and that boron and vanadium have a stronger effect than molybdenum. In marginal applications, a 2-1/4-Cr 1-Mo steel would be expected to behave better than boron-treated or vanadium steels, and practical experience seems to bear this out. 

Whatever the composition of the alloy being used, the stress-relief cracking problem is best solved by avoiding heavy set-through nozzle connections in high-tensile steel. A set-on nozzle or... 

Fig. lOc-f) Their number per unit area increased relatively rapidly with time. Nevertheless the wear-time curves remained linear. Such fissures were not observed when the tissue was worn under constant pressure. Apparently, under these conditions of periodic stress relief, cracks initiating near the tissue s surface do not necessarily propagate to form wear particles. 

Stress-relief-annealing cannot be expected to eliminate SCC in every case. Only residual stresses are reduced in stress-relief-annealing. Applied stresses, which may be responsible for the cracking, will remain. Inhibitors are not 100% effective in combating SCC. Complete coverage and inhibition is difficult to achieve, especially below deposits, in crevices, and in pits. 

Stress relief is of little practical value as a means of preventing stress-corrosion cracking in austenitic steels, as cracking occurs at quite low stress levels even in fully softened material and it is difflcult to ensure that stresses are reduced to a safe level in a real structure. The technique can however be useful in small items but, even in this case, phase changes which reduce stress-corrosion resistance or have other deleterious effects can occur at the stress relieving temperature. 

For carbon steels, however, a full stress-relief heat treatment (580-620°C) has proved effective against stress-corrosion cracking by nitrates, caustic solutions, anhydrous ammonia, cyanides and carbonate solutions containing arsenite. For nitrates, even a low-temperature anneal at 350°C is effective, while for carbonate solution containing arsenite the stress-relief conditions have to be closely controlled for it to be effective . 

In addition, a surprisingly large number of stress-corrosion cracking failures have resulted from the welding of small attachments to vessels and piping after stress-relief heat treatment has been carried out. 

The evidence to date suggests that thermal stress relief prevents cracking in all three environments. 

Silver is often preferred as an undercoat for rhodium by reason of its high electrical conductivity. A further advantage of silver in the case of the thicker rhodium deposits (0-0025 mm) applied to electrical contacts for wear resistance is that the use of a relatively soft undercoat permits some stress relief of the rhodium deposit by plastic deformation of the under-layer, and hence reduces the tendency to cracking , with a corresponding improvement in protective value. Nickel, on the other hand, may be employed to provide a measure of mechanical support, and hence enhanced wear resistance, for a thin rhodium deposit. A nickel undercoating is so used on copper printed connectors, where the thickness of rhodium that may be applied from conventional electrolytes is limited by the tendency of the plating solution to attack the copper/laminate adhesive, and by the lifting effect of internal stress in the rhodium deposit. 

Heat treatment may also affect the extent and distribution of internal stresses. These may be eliminated by appropriate annealing treatments which can remove susceptibility to stress-corrosion cracking. This must be explored in any studies of the performance of materials in environments where stress-corrosion cracking is a hazard. In particular cases, stress-relief annealing treatments may result in the appearance of new phases which, while eliminating the stress-corrosion effects, will induce another type of path of attack. This possibility must be kept in mind in assessing the overall benefits of heat treatments applied primarily for stress relief. 

A recently developed technology takes a different approach to solving the stress problem (14). Rather than chemically plasticizing the sulphur, non-reactive additives are used to holistically plasticize the sulphur concrete mix. In essence these additives are lubricants which operate at the sulphur/aggregate interface to allow slippage and stress relief without disruption and cracking. The apparent permanency of this plasticization approach has been demonstrated by extensive testing over a four year period. 

Determination of residual stress of a failed component is one of the most important steps in failure analysis. The determination of residual stress is useful when failed components experience stress concentration, overload, distortion or the formation of cracks in the absence of applied loads, subjected to corrosive environments as in stress corrosion, mechanical or thermal fatigue due to cyclic loading, or when faults in processing such as shot peening, grinding, milling and improper heat treatment such as stress relief, induction hardening, thermal strains, exposure temperature are involved. 

Corrosion by hydrogen sulfide in partial oxidation plants can be controlled by the use of austenitic steels, but special care to ensure proper stress relief of welds is advisable to avoid stress corrosion cracking in these plants caused by traces of chlorine sometimes present in the feed oil. 

---