Heat treatment stress relief

Heat TVeatment. Stress relief anneal can be accomplished using a full annealing treatment or, in the case of aged material, a stress relief may be obtained by partial reaging at the same aging temperature used initially (see table). 

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. 

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. 

The initial surface composition of boiler tubing, prior to its installation will have an important impact on the amount and type of activated corrosion products in an aqueous reactor coolant. Consequently, the type of thermal pre-treatment the tubing undergoes, for example, for mechanical stress release,will affect the surface oxide film, and ultimately, the corrosion behavior. This particular work has been directed toward characterization of surface oxide films which form on Inconel 600 (nominal composition 77% Ni, 16% Cr, 7% Fe, — a tradename of Inco Metals Ltd., Toronto Canada) and Incoloy 800 (nominal composition 31% Ni, 19% Cr, 48% Fe 2% other, — a tradename of Inco Metals Ltd., Toronto, Canada) heated to temperatures of 500-600°C for periods of up to 1 minute in flowing argon. These are conditions equivalent to those experi enced by CANDU(CANadian Deuterium Uranium)ractor boiler hairpins during in situ stress relief. 

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. 

Stress-relief by heat treatment or shot peening is recommended subsequent to rolling of the tube ends to reduce residual stress to a level at which SCC is less likely to occur. 

To stress relieve or postweld heat treat carbon and low-alloy steels, they are typically heated to 1100°F to 1350°F (595°C to 730°C) for extended time, followed by air coohng. The minimum time is specified by the relevant engineering code, and the temperature must be less than the lower transformation temperature of the steel, which is the lowest temperature at which austenite starts to form, for example, 1333°F (720°C) for plain carbon steels. In order to avoid degrading the required mechanical properties of a heat treated alloy, subsequent fabrication heat treatment temperatures, such as those for stress relief and PWHT, must not exceed the tempering temperature (discussed in the next section). 

As the hquid metal in a weld solidihes, it becomes at least partially constrained by the surrounding parent metal. When the weld metal cools to ambient temperature, the resulting residual stress in the weld is approximately equal to the ambient temperature yield strength of the parent metal. Stress relief heat treatment for welds is called postweld heat treatment. An additional benefit of such treatment is a reduction of the hardnesses of the weld metal and heat-affected zone (HAZ), thns reducing the risk of stress corrosion cracking. 

Austenitic stainless steels are the most significant class of corrosion-resistant alloys for which intergranular corrosion can be a major problem in their satisfactory use. The problem is most often encountered as a result of welding but also may result from stress-relief annealing or incorrect heat treatments. Intergranular corrosion also can occur in ferritic stainless steels and in nickel- and aluminum-base alloys. 

Properly handled, all these steels have good weldability. However, there have been some difficulties in recent years because of cracking in nozzle connections during stress-relief heat treatment. This type of crack has been found in heavy, set-through nozzles in alloy-steel plate over 3-inches thick. It is a particularly undesirable type of failure, first because it occurs when the vessel is almost ready for shipment and, second, because it is difficult to make a satisfactory repair. 

Soft microstructures can be obtained by using steel with low contents of carbon and alloying elements (including manganese) to reduce the hardenability of the HAZ. Additionally, the use of a large weld bead, thin plate, and preheat will reduce the quenching rate in the HAZ. After a bead has been deposited the HAZ can be softened by tempering either as a result of subsequent weld runs or by a postweld heat treatment (PWHT or stress relief). 

Heat treatment given after welding to reduce residual stresses [stress relief heat treatment] and/or to temper hard regions, usually in the HAZ. The temperatures are usually between about 550 and 750 °C,... 

Stress relief heat treatment (see Post-weld heat treatment)... 

Austenitic or duplex stanless steels undergo a solution annealing and water quenching heat-treatment. Additionnal heat-treatments such as for stress relief or post weld treatment are also used to eliminate the internal stresses of the material. 

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