QUESTION & ANSWER
Question
Why is post-weld heat treatment (stress relieving) sometimes necessary for welded vessels?
Answer
During the welding process, the two metal pieces being joined are subject to extreme temperatures and can cause the crystalline structure of the metal to pass through various metallurgical phases. As a result, hardening (and embitterment) of the metal can occur to varying degrees (usually dependent on carbon content). Heat treatment is designed to reduce the hardness in the heat affected zone of the metals and also increase ductility in these sections. Various pressure vessel codes contain the specifics regarding the procedures for post-weld heat treatment.
Heat is usually held for one hour per inch of thickness of the metal. The temperature used is based on the "P-number" of the metals. P-numbers are assigned based on the chemical composition of the metals. Holding temperatures can range from1100-1350 °F (593-732 °C).
Our personnel will supervise any stress relieving operations that the clients desires, making provision for yield at high temperatures and secondary stressing when cooling at the degree and duration of soaking temperature necessary for the material.
HEAT TREATMENT TERMS
STRESS RELIEVING (POSTWELDHEAT TREATMENT)
Is the heating of steel to a relatively high temperature e.g. (590o C -700o C) shortly after welding, and maintaining that temperature for a specific period of time. The purpose is to reduce internal stresses, caused by welding, quench hardening or by cold working. It also has an advantageous effect on the elasticity, ductility and strength of the metal.
PRE - HEATING
Is a very effective means of preventing weld metal or base metal from cracking? It is the application of heat to the base metal prior to a welding or cutting operation. It does not change the properties of the steel, but it is advisable under certain conditions to drive off moisture and help to ensure even expansion to eliminate undesirable stresses. Concurrent heating during welding (also referred to as pre - heating) retards the cooling thus preventing the formulation of an under bead crack.
SOLUTION HEAT TREATMENT
Is performed on certain types of authentic stainless steel to prevent decay in WELDMENT. This is done by the high temperatures (e.g. 1050o C) and the cooling is very rapid.
NORMALIZING
Is much like annealing, but the cooling process is much faster. This result is in increased strength but less ductility in the metal. It also produces higher yield strength with greater tensile strength, and in most instances, a higher impact resistant.
TEMPERING
Is employed after steels have been fully hardened and are too brittle to be of any practical use. Tempering reduces the brittleness and relieves internal stresses. Tempering is actually a heating and slow cooling process. The cooling is accurately controlled to impact certain characteristics to the base metal.
ANNEALING
Was the first form of heat treatment applied to the ferrous metals. It is essentially the softening of the base metal to make it more malleable. The process is involves a controlled heating and slow cooling which includes softness. Additionally, the process removes gasses, relieves stresses and enhances the mechanical properties in alloyed steels. This is generally performed at temperatures up to 700o C
QUENCH HARDENING
Is process of cooling heated metal to room temperature by immersion in liquid or gaseous cooling medium. This method provides greater strength and hardness than slow cooling, but it is accompanied by brittleness.