About heat treatment after welding

Heat treatment as a traditional and effective method to improve and restore metal properties has always been a relatively weak link in the design and manufacture of pressure vessels. The pressure vessel involves four kinds of heat treatment: post-weld heat treatment (stress relief heat treatment); Improve material properties heat treatment; Recovery of material properties heat treatment; Hydrogen removal after welding. This paper mainly discusses the problems of post - welding heat treatment widely used in pressure vessel design.

1) purpose of post-weld heat treatment (stress relief heat treatment)

1. Relax welding to participate in stress

2. Stabilize the shape and size of the structure to reduce distortion.

3. Improve the performance of base material and welding zone, including a. improve the plasticity of weld metal. B. Reduce the hardness of the heat-affected zone. C. Improve fracture toughness. D. Improve fatigue strength. E. Restore or increase the yield strength reduced during cold forming.

4. Improve the ability to resist stress corrosion.

5. Further release harmful gas in weld metal, especially hydrogen, to prevent the occurrence of delayed crack.

2) whether the pressure vessel made of austenitic stainless steel needs post-welding heat treatment

Post-welding heat treatment is to reduce the yield limit of metal materials at high temperature, so as to produce plastic flow where the stress is high, so as to eliminate the residual stress of welding, improve the plasticity and toughness of welding joints and heat-affected zone, and improve the ability to resist stress corrosion. This method is widely used in carbon steel and low alloy steel pressure vessels with body - centered cubic crystal structure. The crystal structure of austenitic stainless steel is face-centered cubic. Because the metal material with face-centered cubic crystal structure has more slip planes than body-centered cubic, it shows good toughness and strain strengthening properties. In addition, in the design of pressure vessels, stainless steel is often used for corrosion prevention and to meet the special requirements of temperature these two purposes, plus stainless steel and carbon steel and low alloy steel is expensive, so its wall thickness will not be very thick. Therefore, considering the safety of normal operation, it is not necessary to put forward the requirement of post-welding heat treatment for pressure vessel made of austenitic stainless steel. It is difficult to consider the deterioration caused by corrosion, material instability, such as fatigue, impact load and other abnormal operating conditions due to use in conventional design. If these conditions exist, relevant scientific and technical personnel (such as: design, use, scientific research and other relevant units) need to conduct in-depth research, comparative experiments, come up with a feasible heat treatment scheme and ensure that the comprehensive use of the pressure vessel performance is not affected. Otherwise, if the need and possibility of heat treatment for austenitic stainless steel pressure vessel are not fully considered, it is often impossible to simply put forward heat treatment requirements for austenitic stainless steel by analogy with carbon steel and low alloy steel.

3) heat treatment of explosive stainless steel composite steel plate container

Explosive stainless steel composite steel plate is widely used in pressure vessel industry because of its excellent corrosion resistance and mechanical strength and reasonable cost performance. However, the heat treatment of this material should also attract the attention of pressure vessel designers.

Pressure vessel designers usually attach more importance to the technical index of composite plate is its combination rate, while the heat treatment of composite plate is seldom considered or should be considered by relevant technical standards and manufacturers.

The explosive processing of metal composite plates is essentially the process of applying energy to the metal surface. Under the action of high-speed pulse, the cladding material inclinedto the base material, and in the state of metal jet, a serrated composite interface was formed between the cladding metal and the base metal to achieve the interatomic bonding.

After the explosive processing of the base metal, in fact, is subjected to a strain strengthening process. As a result, the tensile strength b increased, the plasticity index decreased, and the yield strength value s was not significant. Both the steel products of Q235 series and Q345R show the above strain strengthening phenomenon after testing their mechanical properties after explosive processing.

Therefore, in the current technical standards for the explosive processing of the austenitic stainless steel plate heat treatment has been specified. NB/ t47002.1-2009 "explosive welded composite plate for pressure vessels" part I: the provisions of the stainless steel-steel composite plate are: "the composite steel plate shall be supplied by heat treatment, leveling, cutting (or cutting), and the heat treatment state of the composite plate shall comply with the provisions of the corresponding base material in GB150 or JB4732. According to the requirements of the buyer, the surface of the cladding material can be sandblasted, polished or pickled as specified in the contract.

4) can other methods replace the overall heat treatment of the equipment

Due to the limitations of the manufacturer's conditions and the consideration of economic interests, many people have explored other ways to replace the integral heat treatment of pressure vessels. Although these explorations are beneficial and valuable, they cannot replace the integral heat treatment of pressure vessels at present. The requirement for integral heat treatment has not been relaxed in the standards and procedures currently in force. Among all kinds of schemes to replace the overall heat treatment, the typical ones are: local heat treatment, hammering to eliminate welding residual stress, explosion to eliminate welding residual stress and vibration, hot water bath, etc.

Local heat treatment: it is stipulated in 8.2.6.5 of GB150.4 "pressure vessel" that: "class B, C, D and E welded joints, connection joints of spherical head and cylinder and welding repair parts of defects are allowed to use local heat treatment method." This rule means that the local heat treatment method is not allowed for class A welds on the cylinder, that is, the local heat treatment method is not allowed for the whole equipment, one of the reasons is that the residual stress of the welding can not be eliminated symmetrically.

To eliminate welding residual stress by hammering, a layer of compressive stress is superimposed on the surface of the welded joint by manual hammering, so as to partially offset the adverse effect of residual tensile stress. In principle, this method can inhibit the stress corrosion cracking. However, due to the lack of quantitative indicators and more stringent operating procedures in the practical operation process, coupled with the lack of comparative use of verification work, and was not adopted by the current standard.

Explosive method to eliminate welding residual stress: it is made of explosive into a tape, which sticks to the inner wall of the equipment on the surface of the welding joint, and the mechanism is the same as hammer method to eliminate welding residual stress. It is said that this method can make up for hammering method to eliminate the welding residual stress of some disadvantages, however, there are units in the same two conditions of liquefied petroleum gas storage tank on the overall heat treatment and explosion method were used respectively to eliminate the welding residual stress on contrast test, a year later can examination found that the former welding joint intact, and the method to eliminate the explosion welding residual stress of tank welding joint is many cracks appear. In this way, the explosion method to eliminate welding residual stress, which was popular at that time, is silent.

There are other methods to eliminate welding residual stress that have not been accepted by the pressure vessel industry for a variety of reasons.

In a word, the overall post-welding heat treatment of pressure vessels (including segmental heat treatment in the furnace) is the only acceptable method to eliminate welding residual stress in the pressure vessel industry at present, although it has the disadvantages of large energy consumption, long cycle and various difficulties in the actual operation due to pressure vessel structure and other factors.