Power Heat International Sdn. Bhd. (post weld heat treatment, pwht stress relief, On-site Post Weld Heat Treatment)

What Is Post Weld Heat Treatment

One of the purposes of heat - treating is to relieve stresses caused when two pieces of metal are joint together by welding. As the metal in welded area raised to fusion temperature, the surrounding metal tends to expand but is restrained by the unheated areas adjacent to the welded joint. As a result, the heated portion is place under a compression stress, the magnitude of which may approach and sometimes exceed the yield point of metal.

Upon the cooling, the heated areas contract and set up reverse stresses, with the result that when cooled to room temperature the metal in the weld area is under constant Tensile Stress. To avoid internal stresses, stress-relieving equipment is equally applicable for normalizing, annealing and pre-heating, and electrical heating apparatus is proving successful in carrying out stress relieving requirements, for temperature up to 1100C.

Control of temperature is an important feature and temperature in range of 600C to 950C required to heat-treat the materials can be arrived at a specified rate of temperature increase. The heat can then be adjusted to maintain the area being stress relieved at the temperature and period specified. Reducing the temperature at any given rate controls the cooling period.

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 from 1100-1350 F (593-732 C).

Our personnel will supervise any stress relieving operations that the clients desires, making provision for yiel d 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 (Post Weld Heat Treatment)
- Is the heating of steel to a relatively high temperature e.g. (590 C -700C) 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. 1050C) 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 700C

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.

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 (580C - 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 finished.

Objectives of Heat Treatments

Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is sometimes done inadvertently due to manufacturing processes that either heat or cool the metal such as welding or forming.

Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation. Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics. Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material.

Steels are heat treated for one of the following reasons: Softening, Hardening and Material Modification

Common Heat Treatments

Softening:
Softening is done to reduce strength or hardness, remove residual stresses, improve toughnesss, restore ductility, refine grain size or change the electromagnetic properties of the steel. Restoring ductility or removing residual stresses is a necessary operation when a large amount of cold working is to be performed, such as in a cold-rolling operation or wiredrawing. Annealing full Process, spheroidizing, normalizing and tempering - austempering, martempering are the principal ways by which steel is softened.

Hardening:
Hardening of steels is done to increase the strength and wear properties. One of the pre-requisites for hardening is sufficient carbon and alloy content. If there is sufficient Carbon content then the steel can be directly hardened. Otherwise the surface of the part has to be Carbon enriched using some diffusion treatment hardening techniques

Material Modification:
Heat treatment is used to modify properties of materials in addition to hardening and softening. These processes modify the behavior of the steels in a beneficial manner to maximize service life, e.g., stress relieving, or strength properties, e.g., cryogenic treatment, or some other desirable properties, e.g., spring aging.

Why Use a Controlled Preheat?

Preheating During Welding provides the following benefits:

- Reduces the levels of thermal stress.

- Provides compensation for high heat losses.

- Minimizes the rate of weld hardening.

- Reduced porosity in the weld.

- Reduced hydrogen cracking.

- Improved microstructure of heat affected zone.

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 150C (302F). In practice, it is no more than 100oC (2 12F). 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.

Frequently Asked Questions (FAQs)