One of the essential assets of operation and maintenance in the chemical industries, including the upstream, midstream, and downstream sectors of the petroleum industry, is welding. On many occasions, after a weld occurs, a very popular process occurs. It’s called post-weld heat treatment.
It is considered a handy application, if not done correctly, can cause weakness to equipment by adding residual stresses into the material and reducing the material properties.
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What is Post Weld Heat Treatment (PWHT)?
To ensure that the material strength of the property is not affected after welding, a process known as Post Weld Heat Treatment (PWHT) is continuously employed. PWHT is one of the methods used by welders to reduce residual stresses to combat hardness or enhance material strength.
If PWHT is not adequately performed or is overlooked altogether, the residual stresses plus the load stresses can join together to cause a brittle fracture.
Therefore, it is required to take great care in choosing the right temperature needed for weld-able steel to stop the following problems from occurring, especially when there is an added adjustment of parameters to properties:
- Materials get harder
- Phase changes likely to happen
- The depletion of weld strength may occur.
For hydrogen-induced cracking (HIC) happens as a result of the following parameters.
- Excess amount of hydrogen makes things worse
- Certain base materials are more vulnerable than others. Most times, steels that have a higher concentration of carbon have cracks.
- More intense stresses (both internal and external)
If for any reason, one of the three things listed above is found missing, HIC will not happen.
You might be wondering exactly how post-heating affects the properties of the metal. Increased heat allows the molecules of the metal to expand. This phenomenon allows the smaller hydrogen molecules to diffuse and not concentrate on the seam. So, a lower concentration of hydrogen on and near the seam improves the mechanical properties of the connection.
It’s pretty much obvious that this method has many advantages. However, you must keep in mind that time is not your friend. The weld shouldn’t be allowed to cool to room temperature before you start post-heating. If the temperature is lower than 210F cracking may take place. Those small details make a huge difference and we must not underestimate them.
The maximum temperature and the duration that the metal must be heated, depending on the material and its dimensions. It’s all about allowing hydrogen to move around the metal molecules. It takes some time to achieve a low enough concentration. So, the relative size of the molecules and the way they are connected affect the temperature and duration.
A common question that you might have is why there is maximum temperature since increased temperature almost always affects positively diffusion. You are probably right but there is a big side effect. Too much temperature can change the molecular structure of the metal. This may have a very negative effect on metal properties.
Generally, this healing process takes place around 350˚F – 550˚F. The duration has to do with the thickness of the material. I can say it is a minimum of 60 minutes per inch (25.4 mm).
Since so much detail is important it is better to ask the opinion of an engineer. It’s the only way to be sure enough that you are making this process optimally.
Benefits Of Post-Weld Heat Treatment
In the contemporary world, many things are taken for granted, and people don’t pay enough attention to what it takes to create big scale structures that surround them. For instance, few people know what happens behind the scene to bring on big buildings, ships, and stadiums.
These intricate constructions are built by a dedicated team of skillful welders who make sure these massive projects come alive. They are responsible for miles of welding seams.
A post-weld heat treatment (PWHT) allows welders to maintain toughness on the materials in the weld zone. So the elasticity of the material is increasing and it can be bent but not break. This characteristic is extremely important for the durability of the structure. So if you are interested in knowing how this works, here are some few benefits of post-weld heat treatment.
Softening
After the HAZ material is welded, it gets harder, and with excess hardness, the material becomes nearly useless for a particular kind of work application. This is where PWHT comes to play.
Companies mostly use post-weld heat treatment to change the hard heat-affected zone. This makes this zone stronger and provides adequate protection against brittle fracture failure when working.
PWHT is beneficial because it cools off the weld. The material becomes more durables and it doesn’t crack that easily. This brittle nature of the material is caused by stress corrosion and one of the more common elements, hydrogen. This gas may create cracks during or after the welding process.
When the treatment is added on materials, it makes the metal less rigid and elastic. This is very important since it allows the metal to have better mechanical properties. This fact increases the lifespan of the structure.
A high thermal ingredient may be a significant factor in residual stress during welding. During any thermal welding process, heat may cause an expansion in some metals while in cooling, it may also cause localized contraction. This is what brings about unwanted residual stresses.
To reduce a high-level of stress, the baking of materials is required through the thermal treatment, which should occur at uniform heating at a specific temperature and then restore to room temperature at a specific cooling level.
The extreme cooling and heating experienced during the welding procedures can affect the metal to induce what is called a Heat Affected Zone (HAZ), which is the line between the base metal and the weld metal.
This is the area that withstands stress the most just because of the reaction of heating and cooling that occurs in these regions. The level at which the metal is stressed will ultimately depend on the shape and size of the metal.
In some cases, the stress is already within the metal as a result of the fabrication that the metal undergoes.
The procedures of post-weld heat treatment get rid of some of the stress by making sure the weld and weldment relax and easy to grind and cut.
Prevention of SCC
What does SCC stand for? Stress Corrosion Cracking (SCC) happens because of the force of existing flaws in the material due to applied pressure or load. PWHT helps the prevention of SCC.
Ease Machining
Because PWHT helps to lower the level of hardness, the thermal treatment welded component can easily be machined to bring about dimensional conditions. The softening of these welded parts makes it easy for grinding, machining, including other surfaces treatment that the metal may require.
Avoid Brittle Collapse
One of the reasons why post-weld heat treatment is required on any metal is to improve the mechanical properties of weldment and also make it useful for other specific service conditions. This is demonstrated to help reduce the residual stress level and hardness diffusing the required weldment with some flexibility.
The diffusion improves the resistance and the strength to brittle fracture and also increases the lifespan material for its conditions.
Risks Associated With Post-Weld Heat Treatment
Post Weld Heat Treatment also comes with its own risk. Below we have listed some potential challenges that can be expected when performing PWHT.
Loss Of Strength
Reduced strength of a material can occur when there is too high a temperature or too much time is spent on stress relief post-weld heat treatment. Also, tempering treatment is a cause of reducing strength for tempered and quenched materials.
Therefore, it is essential to make sure that time and temperatures should be controlled to perform a PWHT.
Collapse or Distortion
The tempering temperatures or stress relief happens to materials with significantly lower strength when it’s treated to its height of temperature. If there is any structural component experiencing some kinds of load, then when PWHT is applied, such a structure can encounter distortion or collapse, which can have very high-cost consequences.
This is why it is vital to carefully ensure that all structures that have exposure to high temperatures are adequately supported. Also, if one part of the structure is exposed to a significantly higher level of temperatures than other sections, it could result in a thermal expansion, which can lead to severe distortion or collapse of the structure.
This is a typical situation when shell and tube heat exchangers are made to undergo stress relief post-weld heat treatment. The shell temperature goes up and down in temperature much quicker than the tubes.
Therefore to make sure there is no excessive difference during a PWHT procedure, there should be enough thermocouples attached to the heating components so there won’t be unacceptable temperature gradients.
In some furnaces, there may likely be a flame impingement on some spots on the material, and this could locally increase the temperature to value above the stage transformation temperature, which can result in unexpected phase change and also volume changes.
This has the power to not only lower the mechanical properties but also likely leads to distortion. To avoid this happening, there should be a furnace loading sketches to show the flame impingement prevention and the thermocouples placed on the closest location to the burners of gas or oil controlled furnaces.
Cracking Or Embrittlement
it’s possible to have some heats material with some trace elements to make it susceptible to “temper embrittlement.” During a post-weld heating treatment process, some intermetallic components can be formed that are considered brittle and can lead to major catastrophic embrittlement of the structure.
To avoid this happening, the material should be purchased after testing if materials have likely gone through exposure to the simulated post-weld heat treatment cycle. Some steel materials do come with some retained austenite in their structure due to previous heat treatment they undergo.
After the post-weld heat treatment, the retained austenite can change to martensite like structure that is brittle.
Testing on hardness after the post-weld heat treatment is also essential at knowing if any hardening ever happened during the post-weld heat treatment.
Methods of Performing This Process
Here are three ways to apply the heat for post-weld heat treatment.
Furnace
In this heated environment, the whole component is heated within a furnace while the temperature cycle is applied to the entire part. What this means is that the element shouldn’t be too big as not to enter the furnace.
More so, the component has to be moved to where the furnace is located. However, remote maintenance work and some unwieldy parts can be a bit tasked to move around.
The right side to this type of heat treatment is that the expansion induced by the thermal gradients (which is the uneven temperature in sections of the component) can be significantly lowered.
This is why to achieve this; it is essential to maintain the heating and cooling rates as thicker parts will take a longer time to heat or cool than thinner parts.
Internal Firing
This method involves an insulating component outside while heat is introduced within the element until the whole portion gets heated to the required temperature. This heating process is typically achieved using the gas-fired burners.
This process works best for “hollow” like components like pressure vessels, and it’s a more expensive option of heating.
Locally Applied External Heating
This is ideal for long or elongated components that only needs the heating in some sections rather than having it all over. This method introduces heat to the outside element – although there are times when the components are placed both inside and outside to achieve uniform heating.
The right insulation retains the heat from escaping from the surface through convection and radiation. However, heat will be lost through conduction to unheated parts of the component. This is why it is considered essential to control the thermal gradients from sections experiencing post-weld heat treatment to sections that are still cold.
If this method is not correctly carried out, the variable thermal expansion can bring residual stresses in parts adjacent to the section undergoing PWHT, leading to moving the problem from one component to another.
In Conclusion
If you are interested to learn some basic knowledge on what post-weld heat treatment is and how to manage it, it is advisable to get a hold of a company that is close to where you live and see if you can land yourself a job. Most times, these companies also offer the necessary training to operate their specialized machines.
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