Whether a structure needs to be heat treated after welding is usually decided by the design department after comprehensive consideration of various factors such as the properties of the steel, the thickness of the plate, the manufacturing and use conditions of the structure. For a certain type of product, it is necessary to pass the necessary scientific experiments, and also analyze the accidents that have occurred in the long-term use of such products at home and abroad, and determine under what circumstances the welding residual stress must be eliminated. The following describes some situations in which residual stress relief is required.
Welded containers of various materials have a design wall thickness limit. When the thickness of the container exceeds this limit, it is required to eliminate residual stress to ensure safe use. Especially when it is possible to transport, install, start or use at low temperature, the adverse effects of residual stress need to be carefully considered.
In most cases, heat treatment to relieve residual stress (high temperature tempering or stress relief annealing) should be performed. Otherwise, during the machining process, the internal stress in the structure will be redistributed, resulting in dimensional instability. That is, during the machining process, the distribution state will be constantly changed due to the stress, and the dimensions will also be constantly changing. Such as: locomotive or marine large diesel engine welded structure cylinder block welded structure machine tool parts, etc.
Tempering treatment is required in time after welding. One of its functions is to eliminate welding residual stress and avoid cracks, and it is also beneficial to improve the mechanical properties of welded joints.
Put the whole welded structure into a heating furnace and slowly heat it to a certain temperature. The low carbon steel structure is heated to 600 ~ 650 ℃, and kept for a certain period of time (generally 4 ~ 5 minutes per millimeter of thickness, but not less than 1 h), and then cooled in the air or slowly cooled with the furnace. Considering that the self-weight may cause the bending deformation of the components, the components should be well supported when placed in the furnace.
The overall high temperature tempering has the best effect of eliminating welding residual stress, which can eliminate more than 80% to 90% of the residual stress. It is the most widely used method in production.
The parts with high stress of the welded structure and their surroundings are heated to a relatively high temperature, and then cooled slowly. This can not completely eliminate the welding internal stress, but it can reduce the peak value of the residual internal stress and make the stress distribution more flat to partially eliminate the stress.
The basic principle of this method is to use the uneven heating on the structure to cause an appropriate temperature difference to produce tensile deformation in the weld area, so as to achieve the purpose of eliminating welding stress. The specific method is to make a pair of oxygen-acetylene flame nozzles with a width of 100~150mm and a center distance of 120~270mm on both sides of the weld (Figure Schematic diagram of low temperature welding stress relief) to heat the surface of the component to about 200°C. At a certain distance behind the flame nozzle, water is sprayed for cooling, resulting in a certain temperature difference between the heating zone and the weld zone. Since the temperature on both sides is higher than the weld area, tensile stress is generated in the weld area, so the metal in the weld area is elongated to partially eliminate the tensile stress of the weld. This method is a mechanical method to eliminate internal stress and has been applied in production.
The welded integral steel structure is loaded according to the actual situation, so that the internal stress of the structure is close to the yield strength, and then unloaded, which can partially eliminate the welding stress.
For example, the vessel structure can eliminate part of the residual stress while performing the hydrostatic test.
It should be noted, however, that some residual deformation of the structure will occur after processing with this method.
The internal stress can be reduced by placing the vibrator on the appropriate part of the welded structure for repeated cyclic vibration.
This is a new method, which has the advantages of simple and cheap equipment, low cost, low labor-hour consumption, and no problem of metal oxidation caused by high temperature tempering.
tE-heating zone temperature; tn-weld zone temperature; △t-temperature difference;
1- cooling water pipe; 2- flame nozzle; 3- heating zone