THERMAL AND MECHANICAL TREATMENT OF WELDS
Various thermal and mechanical treatments are often performed on welds to reduce the residual stresses and distortion. They include preheat, postweld thermal treatments, peening, and so forth. These treatments also change the metallurgical properties of weldments.
5.3.1 Reasons for Treatment
• To restore the base properties affected by the welding heat.
• To modify weld-deposit properties.
• To relieve stresses and produce desired micro-structure in base material, HAZ and weld metal.
• The extent of harm the weld has caused determines the subsequent treatment.
• Improve weldability (for example preheat improves weldability).
• To reduce “metallurgical notch” effect resulting from abrupt changes in hardness etc.
• To improve resistance to crack propagation.
5.3.2 Code Requirements
Some welded constructions are required to be in accordance with the recommendations of a code such as the ASME Boilers and Pressure Vessels Code, thermal treatments are specified for certain types of weldments. These recommendations are based upon the existing evidence necessitating the thermal treatment. These are codes for minimum requirements. The fabricator should employ other treatments also based upon his experience in addition to the code requirements. Some important codes are given below for example :
1. ASME Boiler and Pressure Vessels Code, Section I, III, VIII Divs. 1 and 2 (latest edition). New Yorlk: American Society of mecanical Engineers.
2. Code for Pressure Piping, Ansi B 31.1 to B 31.8 (latest edition) New York: American National Standards Institute.
3. Fabrication Welding and Inspection, and Casting Inspection and Repair for Machinery, Piping and Pressure Vessels in Ships of the United States Navy, MIL-STD-278 (Ships) (latest edition) Washington D. C. : Navy Department.
4. General Specification for ships of the United States Navy, spec. 59-1 (latest edition) Washington D. C. : Navy Department.
5. Rules for Building and Classing Steel Vessels (latest edition) New York : American Bureau of Shipping.
6. Structure Welding Code AWS D 1.1 (latest edition as revised). Miami : American Welding Society.
7. United States Coast Guard Marine Engineering Regulations and Materials, spec. CG - 115 (latest edition). Washington D. C. : United States Coast Guard.
As these documents are constantly revised, the latest available versions should be obtained and followed.
5.3.3 Common Thermal Treatments
Preheat. Preheat temperatures may be as low as 26°C for out door welding in winter to 650oC when welding ductile cast iron and 315oC when welding highly hardenable steels. In many situations the temperature of preheat must be carefully controlled. The best way is to heat the part in a furnace and held at the desired temperature.
• Preheating is very effective means of reducing weld metal and base metal cracking. It retards the cooling rates and reduces the magnitude of shrinkage stresses.
• Also the thermal conductivity reduces as temperature increases (for iron thermal conductivity at 595c is 50% of its value at room temperature). This also reduces the cooling rate resulting in favourable metallurgical structure, HAZ also remains at the transformation temperature for a longer period of time permitting the formation of ferrite and pearlite or bainite instead of martensite.
• When an area being welded is under severe restraint, localized preheat may increase the amount of shrinking and cause cracking.
Thus preheat must be used with caution, since detrimental effects may result under certain conditions.
Electrical strip heaters are commonly used on site for preheating.
These must be properly insulated to avoid danger of shock to welders.
Induction heating, using 60 Hz (or 50 Hz) transformers of suitable capacities built for this purpose, is a common method of preheating pipe joints for welding.