Because preheating helps prevent cracking and other problems that can result in costly rework. Typically, welding preheat is applied to pipes or plates that are 1 inch thick or thicker before welding. Preheating the weldable base material minimises the temperature difference between the material and the arc, slows the cooling rate in the weld area, and reduces the danger of cracking.
The required for preheat increases with steel thickness, weld constraint, steel carbon/alloy content, and weld metal diffusible hydrogen. Preheating is frequently accomplished using fuel gas torches or electrical resistance heaters.
Although it may appear to be an expensive operation, it can save time and money in the long run by lowering the likelihood of a faulty weld.
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Preheating serves the following purposes:
- Reduce the possibility of hydrogen cracking
- Reduce the weld heat affected zone’s hardness
- Reduce shrinkage stresses during cooling and enhance residual stress distribution
- If preheat is applied locally, it must reach at least 75mm from the weld point and be measured preferably on the opposite face to the one being welded
Oil and gas, power plants, structural fabrication, transmission pipelines, and ship building are among the industries that frequently demand the benefits of preheat treatment in both shop and field applications.
Welded Joint Heat Treatment
Heat treatment is a time-consuming and expensive process. It can alter the strength and toughness of a welded joint, as well as its corrosion resistance and residual stress level, but it is also a required process stated in many application regulations and standards.
Furthermore, it is an important variable in welding technique qualification criteria. Before examining the numerous heat treatments that a metal can be exposed to, it is necessary to clearly identify the various words used to describe the various heat treatments that can be given to a welded joint.
Such phrases are frequently misused, especially by non-specialists; for a metallurgist, they have very specific connotations.
The optimal approach for a certain application is determined by task variables such as material thickness, size, project timetable, budget, and available personnel knowledge level.
Treatment with a solution
Performed at a high temperature and aimed to dissolve elements and compounds that are then maintained in solution by rapidly cooling from the solution treatment temperature.
This may be done to weaken the joint or to improve its corrosion resistance. It may be followed by a lower temperature heat treatment with particular alloys to reconstruct the precipitates in a controlled manner (age or precipitation hardening).
Annealing
This involves heating a metal to a high temperature, when recrystallisation and/or phase transformation occur, and then slowly cooling it, often in a heat treatment furnace.
This is frequently used to soften the metal after it has been hardened, for example, by cold working; a thorough anneal results in the softest microstructures.
It also causes a decrease in yield and tensile strength, as well as, in the case of ferritic steels, a decrease in toughness.
Normalizing
This is a thermal treatment that is only used on ferritic steels. It entails heating the steel to 30-50°Cover the upper transformation temperature (approximately 910°C for a 0.20% carbon steel) and cooling in still air. As a result, grain size is reduced and strength and hardness are improved.
Quenching
This entails rapidly cooling from a high temperature. To make an extremely high strength, fine grained martensite, a ferritic steel would be heated above the upper transition temperature and quenched in water, oil, or air blast. Steels are never used in the quenched state; they are always tempered after the quenching process.
Tempering
A heat treatment performed on ferritic steels at a relatively low temperature, below the lower transformation temperature; in the case of a standard structural carbon steel, this would be in the 600-650°C range.
It decreases hardness, decreases tensile strength, and increases ductility and toughness. Most normalized steels are tempered before welding, and all quenched steels are quenched and tempered.
Precipitation Hardening Or Ageing
A low temperature heat treatment aimed to produce precipitates with the proper size and distribution, enhancing yield and tensile strength.
In most cases, it is preceded by a solution heat treatment. Steel may be aged at temperatures ranging from 450 to 740 degrees Celsius, whereas aluminum alloys are aged at temperatures ranging from 100 to 200 degrees Celsius.
Longer times and/or higher temperatures cause the precipitate to grow in size and lose hardness and strength. Stress reduction This is a heat treatment designed to eliminate residual stresses caused by weld shrinkage, as the name implies.
It is based on the fact that when the metal’s temperature rises, the yield strength falls, allowing residual stresses to be transferred via weld and parent metal creep. Cooling from the stress release temperature is managed to avoid damaging thermal gradients.
Post Heat
A low temperature heat treatment performed immediately after welding by increasing the preheat temperature by 100°C and holding this temperature for 3 or 4 hours.
This aids in the diffusion of any hydrogen in the weld or heat-affected zones out of the joint, lowering the danger of hydrogen-induced cold cracking.
It is only utilized on ferritic steels where hydrogen cold cracking is a serious problem, such as exceptionally crack sensitive steels and extremely thick joints.
Heat Treatment After Welding (PWHT)
So, what exactly is ‘post weld heat treatment’? To some engineers, it is a very ambiguous phrase that refers to any heat treatment performed after welding is completed.
Others, however, have a very specific meaning, particularly those functioning in line with pressure vessel regulations such as BS PD 5500, EN 13445, or ASME VIII.
It is therefore recommended when an engineer discusses post-weld heat treatment, annealing, tempering, or stress reduction. Heat treatment after welding may be performed for one or more of three primary reasons:
- To achieve dimensional stability in order to maintain tolerances during machining operations or during service shake-down to produce specific metallurgical structures to achieve the required mechanical properties to reduce the risk of in-service problems such as stress corrosion or brittle fracture by reducing residual stress in the welded component.
- The range of heat treatments available to achieve one or more of these three goals in the ferrous and non-ferrous metals and alloys that can be welded is obviously far too broad to address in depth in our brief Job Knowledge pieces.
- The next part will focus on the PWHT of carbon and low alloy steels as needed by application standards, with a brief mention of additional types of heat treatment that the welding engineer may encounter in ferrous alloys.
- There are two primary mechanisms at work here: stress release and microstructural alterations or tempering. Stress Reduction: What Is the Purpose Of Stress Reduction?
It is an expensive technique that necessitates heating a portion or all of the welded object to a high temperature, and it may result in unwanted metallurgical changes in some metals. As previously said, it could be one or several reasons.
Because of the large residual stresses locked into a welded joint, deformation beyond allowable dimensions may occur when the item is machined or placed in service.
- In carbon and low alloy steels, high residual stresses can enhance the likelihood of brittle fracture by acting as a driving force for crack propagation.
- In the right setting, residual tensions will cause stress corrosion cracking, such as carbon and low alloy steels in caustic service or stainless steel exposed to chlorides.
- What is the source of these high residual stresses? Welding is the process by which molten metal is deposited between two substantially cool parent metal faces.
- The weld metal contracts as the joint cools but is restricted by the cold metal on either side; the residual stress in the joint thus increases as the temperature drops.
- When the stress reaches a critical level (the yield point or proof strength at that temperature), the metal plastically deforms via a creep mechanism, causing the stress in the joint to meet the yield strength.
As the temperature falls, the yield strength increases, hindering deformation, to the point that the residual stress is frequently equivalent to the proof strength at room temperature.
The component is warmed to a sufficiently high temperature to reduce this high degree of residual stress.
As the temperature rises, the proof strength decreases, allowing deformation and residual stress to reduce to an acceptable level.
The component would be retained at this temperature (soaked) for a length of time until a stable condition is achieved, after which it would be cooled to room temperature.
The remaining residual stress in the joint is equal to the proof strength at the soak temperature.
Figure 1 indicates that residual stress in a carbon manganese steel declines rather regularly from ambient to roughly 600 degrees Celsius, but that residual stress in high strength creep resistant steels must be above 400 degrees Celsius before it begins to reduce.
Stainless steel is unaffected until temperatures approach 500 degrees Celsius. As a result, there is a range of soak temperatures for the various alloys to obtain an acceptable reduction in residual stress without impairing the joint’s mechanical qualities.
This temperature will be between 550-620 degrees Celsius in carbon manganese steels, 650-750 degrees Celsius in creep resistant steels, and 800-850 degrees Celsius in stainless steels.
Frequently Asked Questions (FAQs)
When Should Steel Be Preheated Before Welding?
Welding preheat is typically used before welding 1 inch or thicker steel or steel alloy pipes or plates. Preheating is frequently required in shop and field welding for applications such as oil and gas, transmission pipelines, power plants, structural construction, mining, shipbuilding, and heavy equipment.
Is Preheating the Base Material Usually Required Before Welding?
The PWHT is determined by the thickness of the repair. If the thickness of the repair welding is equal to or more than the minimum base metal thickness that needs PWHT, PWHT must be performed after the repair is completed.
What Is the Objective of Welding Preheating and Post heating?
Post-heating is the continuation of preheat after the weld has been completed to allow for higher rates of hydrogen evolution from the weld. The post-heat temperature can be the same as, or higher than, the preheat temperature.
What Type of Gas Is Utilized for Preheating?
LPG is utilized for the great majority of preheating in the welding industry. LPG has the following advantages: I It has better total heat combustion.
Is It Possible to Weld Without Using Heat?
Cold welding, also known as contact welding, is a type of solid-state welding that uses little or no heat or fusion to link two or more metals together. The energy used to create a weld, on the other hand, is in the form of pressure.
Why Is It Necessary to Preheat When Welding Cast Iron?
Preheating the cast iron item before welding will slow the rate of cooling of the weld and the surrounding region. If feasible, it is always preferable to heat the entire casting. Typical preheat temperatures range from 500 to 1200 degrees Fahrenheit.
What Is the Role of Heat in Welding?
Heat input is significant because it influences weld cooling rates, which influence the heat affected zone and the microstructure of welded materials.
Changes in the microstructure of welded materials can have an immediate and direct impact on the weld’s quality and mechanical qualities.
What Is Welding Preheat Temperature?
The same source defines the preheat temperature as “the temperature of the base metal in the volume surrounding the point of welding just before welding begins.” It is also the temperature just before the second and subsequent passes in a multipass weld” (Interpass Temperature).
What Type Of Flame Is Used For Preheating?
LPG (liquefied petroleum gas) (liquefied petroleum gas)
For heating thicker weld metals, LPG is recommended. The oxygen-to-LPG blend ratio is 4:1, which greatly increases oxygen consumption as well as noise levels. The big flame also increases the heat demand on the operator.
Why Do We Heat the Gas?
Natural gas preheating raises the temperature of the gas sufficiently to allow it to reach the optimal needed temperature following pressure throttling and temperature reduction.
Which Heating Methods Are Used in Welding?
Overview. A heat source, such as an arc, plasma, torch, laser, or electron beam, is used in most welding procedures.
Which Material Preheating Is Required for Welding?
Preheating is frequently used when welding cast iron, high carbon steel, or alloy steel because it reduces the cooling rate of the parent metal close to the weld and the weld itself, preventing the development of martensite, which accounts for hardness throughout the weld.
How Long Does Preheating Take? 12-15 Minutes
Preheating an oven takes roughly 12-15 minutes to reach 350oF, though this time may vary depending on the kind and size of your oven. Preheating your oven is critical to achieving excellent culinary outcomes.
How Long Should Preheating Take?
Preheating your oven for 8-10 minutes is usually sufficient to bring it up to the temperature of 160-180 degrees Celsius, at which most baking is done. However, if you are welding at a higher temperature, such as 200°C, you need extend the preheating time by 4-5 minutes.