For high-quality welding projects, post-weld heat treatment is an essential part of the process. These specific heat treatments drastically decrease the effects of thermal stress that are created during the welding process. Thermal stress in the Heat Affected Zone occurs because metal, despite its strength and durability, is vulnerable to the high temperatures generated during the welding process. Let’s take a closer look at post-weld heat treatments, or PWHTs as they are sometimes called, and why they are important.
The Importance of Top-Quality Welding
Welding is an indispensable tool that allows for the construction and manufacture of piping, pressure vessels, pressure-containing parts, and structural members. During this process, the molten pool often experiences sudden and extreme temperature increases. As it begins to cool, the metal will be subject to shrinking caused by the sudden thermal stress. This can have a negative impact on quality and chemically alter the structure of the structure of the steel. If this problem is not addressed, the weldment can fail to perform its function, potentially leading to disastrous effects.
What Causes Thermal Stress?
There are many causes of thermal stress, but fortunately they can be largely reversed or eliminated with the effective application of post-weld heat treatments. Some of the reasons why thermal stress may occur include the following:
- Layers of metal cause a thermal gradient to form across various sections of the weld.
- Multiple areas of the metal are affected.
- The majority of the steel is at or around ambient temperature.
- Rapid cooling results change the macrostructure of the steel; this can make the steel weaker and less effective.
A weld heat treatment involving controlled heating and cooling of the weld area is the key to eradicating these potential issues. This will give the structure of the steel adequate time to return to its original state. The professionals at STI Group, equipped with top-notch welding supplies, have unrivaled experience in this process.
Technical Specifications for PWHT
Temperature Ranges
PWHT typically involves reheating the welded material to temperatures ranging from approximately 300°F to 1,125°F (150°C to 600°C). For carbon steel, the treatment is performed below the lower critical temperature, which is around 1,333°F. This specific range is crucial to prevent the formation of a hard martensite structure, which can render the material brittle.
Material Thickness and PWHT Requirement
Carbon Steel and Low-Temperature Carbon Steel (LTCS) with a thickness of at least 0.75 inches (20MM) generally require PWHT. Low Alloy Steel’s requirement for PWHT varies based on its chromium content and thickness. For those containing less than 0.5% Chromium, the thickness threshold is above 0.75 inches (20MM), and for those with more than 0.5% Chromium, it is above 0.50 inches (13MM).
Cooling Rate
The cooling rate after PWHT is a critical factor. Controlled cooling is essential to maintain the desired material properties and to avoid introducing brittleness in the material.
Factors for Stress
The quality of the metal is greatly influenced by the rate at which is heated and cooled. Post-weld heat treatments work by lowering the temperature of the metal back down to its original level. The thicker the metal, the more susceptible it is to residual stresses, which can compromise the structural integrity of the weld. Other factors that affect the stress level of steel include:
- When two different materials are welded together, their structures have the capacity to change the structure of the base metal.
- If hydrogen is introduced during the welding process, the metal becomes more susceptible to stress, corrosion, cracking, and rusting.
- If the weld area is harder than the rest of the steel, the potential for corrosion and shattering increases.
- When weld parts are shaped differently, thermal gradients can form across various parts of the steel.
How Post-Welding Heat Treatments Work
After welding is complete, post-weld heat treatments, or PWHT, heat the metal using a very controlled process. The temperature is lowered below the initial transformation point, and the metal then soaks at that temperature for an extended period of time. Depending on the chemical properties of the metal, temperatures will range from 595˚C to 775˚C. For each additional inch of thickness, roughly about 15 minutes of additional soaking time will be needed.
Post-Welding Heat Treatments Yield Dependable Results
In short, post-weld heat treatments are one of the most critical engineering processes used in the steel manufacturing and fabrication industry. Without this essential technique, ensuring the optimal mechanical properties of the metal at the highest quality level would be challenging. Pressure vessels and other structural steel equipment cannot be allowed to fail. However, by using post-welding treatments, metal structures and vessels can be created that will safely last for years to come.
