Preheating and Post Weld Heat Treatment
Technology InformationStress relieving is the common term used to define a process of heat treatment introduced wherever metal structures are joined or extended by welding. Localized stresses are caused by the expansion and contraction of the hot welding. Proper heat treatment is applied to spread these stresses over a much wider area thus the metallurgical structure.
Stress forces can be reduced during the welding process by preheating. The parts to be joined are brought up to a temperature consistent with the hot weld at its highest expansion level, the whole operation then cools off at the same rate so reducing residual stresses. Preheating can prevent a route fracture when a second run of weld is introduced while the first weld is kept at a high temperature.
Preheating and concurrent heating are concerned in direct liaison with the welding process. Post weld heat treatment is a more usual occurrence and the accepted condition in reference to Stress Relieving. The principle is similar in that the homogeneous metal of the two parts and the deposited metal of the weld must be brought up to a high temperature and allowed to cool off at an equal rate to spread the inherent stresses from the welding process. Post heat treatment is carried out more critically depending on the metallurgical properties, thus keeping base metal below the “Transformation Stage.” Post heat treatment can be applied to involve annealing, normalizing, tempering, solution heat treatment, etc. With localized heating caused by welding, hot forming or flame cutting, stress corrosion cracking can occur. Non-uniformity of heat treatment can do more harm than good and increase residual stress. Properly controlled heating equipment is therefore vital.
Historically, stress relieving was attempted and by pure chance achieved success by dropping welded parts into a hot fire. Today it is quite a different consideration and far from being that simple. Major breakdowns and even loss of life have been attributed to the ignorance of proper stress relieving to an extent that it has become an industry of complex standards in technology. Insurance companies are wise to the added safety margin that stress relieving provides and look favorably to where this process becomes standard practice in the construction of pressure vessels, piping, tanks, weldments, etc.
Various methods of stress relieving are in current use such as chemical packs of combustible material, gas powered appliances, electric induction coils and electrical resistance elements. It is the latter which in most cases serves the purpose best, since complete control of the equipment is possible, moreover electrical resistance is versatile and more manageable from all aspects of set up and handling.
Correct stress relieving demands a pre-calculated controlled rate of heat rise, maximum temperature soak period and cooling rate which varies according to metal composition, the volume and mass of the workpiece and so on. The determination of the correct theoretical formula makes the practical achievement an art for the experienced stress relieving technician alone.
Modern furnaces of course are the ideal stress relieving medium. Quite obviously it is not generally worthwhile or possible to take work pieces to a furnace or vice versa, however, temporary furnaces are constructed around structures on site and often the work piece can act as its own furnace when suitably insulated. The use of high velocity gas burners in a lagged sphere are an example of this.
When the removal of residual stresses is of primary importance, it may be desired to combat failure due to mechanical or thermal overloading , brittle fracture conditions at low temperatures, corrosion and cracking or distortion.
Desired metallurgical changes can be important in the homogenization in the metallurgical structure of the solution and stabilization of specific metallurgical phases, age hardening, formulation of specific structures. Cold cracking is prevented by the removal of hydrogen.
Stress relieving has brought about a complete new independent industry of specialist organisations, mostly because it is recognised as a broader science than was previously realised. Rapid changes are taking place daily in modern stress relieving techniques with the development of more sophisticated equipment to ensure the more reliable results now sought after by the welding and engineering construction field. Industrial Heating is one such company adding to the progress of stress relieving. Close examination of our products and services will determine the extent of the research that has been exploited.