Preheating is a commonly used technique in welding. It involves heating the part to be welded to a temperature above ambient temperature before or during the welding process.
Most modern specifications require specific preheat temperature ranges depending on the standard and type of material being welded.
In this article, we will discuss the importance of proper preheating, its benefits, and the consequences of improper preheating using examples.
1. Techniques
Preheating is the process of heating the part to be welded to a temperature higher than ambient temperature, before or during welding.
Preheating is a mandatory requirement in welding, and specific preheating temperature ranges are described in the pre- and post-welding specifications. However, under certain conditions, alternative preheating methods may also be used.
Preheating offers several advantages, regardless of whether it is mandatory or not:
- Reduces the shrinkage stress between the weld and the adjacent base metal, which is especially crucial for high-stress welds.
- Slows the cooling rate during weld cooling in the main temperature range, preventing excessive hardening and reducing weld and heat affected zone (HAZ) ductility.
- Slows the rate of cooling in the 400°F temperature range, allowing more time for hydrogen to escape from the weld and adjacent base metal, thereby preventing hydrogen-induced cracking.
- Removes contaminants.
The amount of preheat required for welding is not determined solely by the minimum standard described in the specification. Instead, one or more of the following methods can be used:
- Calculation tables
- Carbon equivalent estimation
- Crack parameter estimation
- Spark Test Estimation
- Rule of thumb
The preheat temperature range is generally suitable for various sizes and restrictions of weld grooves.
Although many specifications specify a minimum preheat temperature, in some cases a lower preheat temperature may be used, while in others a higher preheat temperature may be required.
2. Spreadsheet
There are several “preheat calculation tables” available that use linear or circular rulers to determine the preheat temperature. These tables allow you to predict the required preheating temperature based on material identification and base metal thickness.
3. Carbon equivalent
Carbon equivalent (CE) is a useful measurement for determining whether preheating is necessary and to what extent. Here are the guidelines:
- If EC is less than or equal to 0.45%, preheating is optional.
- If CE is between 0.45% and 0.60%, the preheat temperature range should be between 200°F and 400°F (100°C to 200°C).
- If CE is greater than 0.60%, the preheat temperature range must be between 400°F and 700°F (200°C to 350°C).
If the CE is greater than 0.5, it is advisable to delay final non-destructive testing (NDE) for at least 24 hours to determine if any delayed cracks are present.
4. Crack parameters
The Ito & Bessyo Parameter Crack Detection (PCM) method can be used when the carbon equivalent is equal to or less than 0.17% by weight or when high strength steel is used. This approach is useful for accurately determining when preheating is necessary, as well as when to apply forced preheating and what temperature to use. Here are the guidelines:
- If the PCM is less than or equal to 0.15%, preheating is optional.
- If PCM is between 0.15% and 0.26-0.28%, preheat to a temperature range of 100°C to 200°C (200°F to 400°F).
- If PCM is greater than 0.26-0.28%, preheat to a temperature range of 200°C to 350°C (400°F to 700°F).
5. Spark Test
Spark testing has been used for many years as a method for estimating the carbon content in carbon steel. The quality of the spark produced indicates the level of carbon content, with higher carbon content resulting in better spark and greater need for preheating.
Although this method is not the most accurate, it is simple and can give a general indication of the required preheat temperature. By examining the quality of the spark produced, the relative level of preheat temperature required can be determined.
6. Rules of thumb
Another effective but less accurate method for selecting the preheat temperature is to increase it by 100°F (50°C) for every 10 points based on carbon content (0.10% by weight). For example, if the carbon content is 0.25% by weight, the preheat temperature must be at least 250°F (125°C) or higher.
However, if there are coatings or other components close to the weld, the preheat temperature specified in the original production specification may not be appropriate.
If the welding heat input is close to the maximum range allowed by the standard process, the heat transferred to the welded components may be sufficient to balance the need for preheating. As a result, the affected metal may be heated to or above the minimum preheating requirements. In these cases, external methods can be used to reduce preheating requirements.
It should be noted that this approach involves imprecise ranges and conversions (e.g., °F to °C) as preheating is not an exact science.
In many cases, it is also common to continually increase the preheating temperature until the problem, such as cracks disappearing, is resolved.
On the other hand, in some specific situations, it may be possible to achieve the desired objective even if the preheating temperature is lower than the recommended value or the temperature specified in the production specification.
7. Practical application
To avoid material softening caused by preheating, it is important to pay attention to actual operation skills.
Choose welding processes and electrodes that rarely introduce hydrogen.
There are certain techniques that can help reduce or alleviate residual stress.
Careful monitoring is required to ensure correct use of the preheating method.
The following descriptions are crucial to the successful implementation of these techniques.
8. Welding slot size and abilities
Welding skills have a significant impact on welding shrinkage, residual stress, heat input control and crack prevention.
Short welds have less longitudinal contraction than long welds.
Reverse welding or special welding sequences can be employed to reduce residual stresses.
Heat input must be controlled or reduced.
Linear welds with small oscillations should be used instead of those with large oscillations.
9. Reduce cracking
Appropriate manufacturing processes can help reduce or eliminate weld craters and cracks.
- Compared to welds with thin and wide sections, those with circular sections tend to have fewer cracks.
- Sudden starts or stops in welding must be avoided. Welding operations and weld formation can be controlled by up/down slope welding techniques or through electrical means using the welding power source.
- Sufficient deposited materials must be used to prevent cracking caused by welding contraction or normal welding.
Based on experience, to avoid cracking due to insufficient material deposited in the weld (which is also a requirement in many production specifications), the amount of metal deposited must be at least 3/8 in. (10 mm) or 25% of the weld groove thickness. .
10. Preheating method
In workshops or fields, preheating can be achieved using flame heating (air-fuel or acetylene fuel), resistance heating, electronic induction heating, and other methods.
Regardless of the method used, preheating must be uniform.
Unless there are specific requirements, preheating must penetrate the entire thickness of the weldment.
Figure 1 shows equipment that uses resistance heating (without insulation, later application) and induction heating.
Fig. 1 – resistance heating (left) and induction heating (right)
11. Preheat monitoring
Various devices can be used to measure and monitor temperature.
The components or weldments to be welded must be preheated until the material is fully saturated with heat.
Whenever possible, the degree of thermal penetration should be tested or evaluated.
For most welding applications, it is usually sufficient to monitor the temperature at a distance from the weld edge.
Temperature monitoring or reading must not contaminate the welding groove.
12. Temperature indicator pen
Indicator pens or pencil-like tools are used to determine the minimum temperature reached during preheating. These tools melt at a specific temperature, which allows for a simple and cost-effective method of determining the melting temperature of the pen.
However, if the temperature of the part exceeds the melting temperature of the indicator pen, it will not work properly. In these cases, it may be necessary to use multiple indicator pens with varying melting temperatures to ensure accurate temperature readings.
13. Electronic temperature monitoring
For preheating and welding operations, direct measuring equipment such as contact pyrometers or direct reading thermocouples with analog or digital readings can also be used. These instruments must be calibrated or have their ability to measure the temperature range verified in some way.
The thermocouple, in particular, has the advantage of continuously monitoring and storing data. As a result, it can be used with a curve recorder or data acquisition system during preheating or post-weld heat treatment (PWHT) operations.
The American Welding Society (AWS) D10.10 provides several diagrams and examples of appropriate thermocouple placement positions.
14. Monitoring of “indigenous rights”
For many years, various “indigenous methods” have been used to determine whether the preheat temperature is sufficient. One such method is to spray saliva or smoke directly onto the workpiece. The sound produced by saliva is used as a temperature indicator, although this method is not very accurate. Some experienced individuals still use this technique.
A more accurate way to determine the preheat temperature is to use an acetylene torch. The flame is adjusted to produce high carbonization, creating a layer of gray smoke in the area to be preheated. The welding torch is then adjusted to produce medium smoke and is used to heat the gray smoke area. When the gray smoke disappears, it indicates that the surface temperature has reached more than 200°C (400°F).
It is important to ensure that the preheating temperature is reached throughout the thickness of the part and in the welding area. Most monitoring is just for the outer surface of the workpiece, but AWS D10.10 provides best practices for the immersion zone and requires that the entire thickness of the workpiece be heated during pipe welding.
Careful observation is required during preheating to avoid overheating the base metal, especially when using resistance or induction heating methods. Many conveyors now require thermocouples to be placed under each resistance heating plate or induction coil assembly to monitor and prevent overheating.
15. Summary
Regardless of whether preheating is required and the preheating method used, preheating provides several benefits, including:
- Reduction of shrinkage stress in the weld and adjacent base metal, which is particularly beneficial for welded joints under high restraint;
- Slow down the cooling rate of the part in the critical temperature range, avoiding excessive hardening and reducing the softening of the weld and the thermally affected zone (HAZ);
- Allow more time for hydrogen to diffuse from the weld and adjacent base metal, slowing the rate of cooling as the part passes through the 400°F (200°C) temperature range, which helps prevent hydrogen-induced cracking ;
- Decontamination of the part;
When preheating, it is best to evenly heat the entire welding thickness to the specified preheating temperature. Overheating of a local area can cause material damage, so it should be avoided as much as possible.
