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.
DEFINITIONS
Post-weld heat treatment
Heat treatment carried out after welding in order to decrease residual welding stress and/or change the micro structure.
Post-forming heat treatment
Heat treatment carried out after forming in order to regain the original properties of the material or reduce stresses caused by the forming.
Holding temperature
The holding temperature is the temperature range in which the product or component is kept in order to achieve specified properties. The holding temperature depends on the type of heat treatment and material. Normally it is expressed as a temperature range (580 – 620 °C).
Holding time
Holding time is the time the product or component is kept at the holding temperature. The holding time starts when the temperature at all the measuring points has reached minimum value of the range of the holding temperature and stops when one of the measuring points falls below that temperature. The duration of the holding time depends on the type of heat treatment, material and material thickness.
Unloading temperature
The temperature of the product or component when it is taken out e.g. of a furnace or when any other heat treatment is.
.A competent own heat treatment technology is a key factorfor high product quality."
Heat treatment is a process based on a highly specialised technology aimed at giving metals specials properties on the surface and in depth - as high strength and hardness. For us, heat treatments are therefore essential for making our gearbox components capable of withstanding the stress levels that are forecast in the design stage, ensuring superior quality standards during the total operation time.The outsourcing of these processes, a prevailing practice among some gearbox manufacturers, frequently entails logistic problems connected with transport, storage and off-site processing of workpieces and this leads to longer delivery times and higher costs.Third-party processing firms may gauge their treatments on average or standard values that can't be everytime the optimum for the gears of a gearbox.
This is the reason why Haba Stress Engineering carries out its heat treatment at its own premises in own facilities, adopting the specific standards that are ideal for its products, thus avoiding all inconveniencies of external processing.
The same heat treatment processes and procedures applied to our gears and components are successfully used for commercial heat treatment jobs, so that the highest customers quality requirements and specifications can be guaranteed To minimize residual stress, it is crucial that the suction roll shell undergoes heat treatment and is cooled in a controlled manner. The heat treatment process also ensures that the weld joints have the same excellent properties as the rest of the material.
Vertical heat treatment
The suction roll shell undergoes heat treatment in a vertical position in the furnace to ensure that its round shape is not distorted by its own weight. The entire heat treatment process can be divided into three stages: heating, holding time and cooling. The cooling stage is crucial for obtaining the right properties. A uniform cooling rate is achieved by blowing air along the inside of the suction roll shell during the cooling process. The difference in temperature between the inside and outside of the suction roll shell must never exceed 150oC (302°F). In practice, it is no more than 100oC (2 12°F). The entire process of heating, holding time and cooling is controlled by thermal couples placed around the inner and outer diameter of the suction roll shell along its entire length.
Compressive stress on the inner and outer walls
The controlled heat treatment process improves the service life of the suction roll shell. By controlling the cooling process so that the temperature on both the inside and outside of the suction roll shell is constantly lower than that at its core, a compressive stress is created on the inside and outside of the shell. Compressive stress means that the suction roll shell can handle greater loads and is more resistant to corrosion fatigue in those parts where cracking is otherwise most likely to occur.