Pre-Heating

Preheating in welding is the process of heating the base metal (the material being welded) prior to starting the welding process. This is typically done to reduce the risk of cracking in the weld and to improve the quality of the weld.

Table of Contents 1.1   Why Pre-Heat Required?  1.2     Types of Pre-Heating methods? 1.3   When Pre-heating required? 1.4      Advantages of Pre-heating? 1.5      Metal Temperature Measuring While Preheating? 1.6      Pre-heating Requirement As per ASME B31.3

Preheating is commonly used in welding of thick sections of steel or in materials that have high carbon or alloy content. When these materials are welded, they can experience high thermal stresses, which can cause cracking in the weld and the surrounding base metal.

The preheating temperature will depend on the thickness and composition of the base metal, as well as the welding process being used. In some cases, post-weld heat treatment may also be necessary to further reduce the risk of cracking and improve the properties of the weld.

 

1.1 Why Pre-Heat Required?

Preheating is often required for welding because it helps to prevent cracking in the welded joint and improve the quality of the weld. There are several reasons why preheating is necessary in welding:

 a)   Reduce thermal stress: Welding involves high temperatures, and when the welded joint cools, the metal contracts, causing thermal stresses. Preheating helps to reduce these thermal stresses and prevent cracking in the welded joint.

 b)   Improve weld quality: Preheating can also improve the quality of the weld by ensuring that the joint is at a consistent temperature throughout the welding process. This can help to reduce porosity, increase weld penetration, and produce a stronger and more reliable weld.

 c)    Avoid hydrogen cracking: Preheating can also help to prevent hydrogen cracking, which occurs when hydrogen is absorbed into the weld and causes cracks to form. Preheating can help to drive out any moisture or hydrogen that may be present in the base metal, reducing the risk of cracking.

 d)   Address material properties: Some materials, such as high-strength steels, are more prone to cracking during welding. Preheating can help to address these material properties by reducing the cooling rate and ensuring that the welded joint cools at a slower rate, reducing the risk of cracking.

 

1.2  Types of Pre-Heating methods?

There are several types of preheating that can be used in welding, including:

1.     Torch preheating: This involves using a flame torch to heat the base metal prior to welding. The torch can be fuelled by gases such as acetylene, propane, or natural gas.

2.     Induction preheating: This method uses an electromagnetic field to heat the base metal. Induction preheating is often used for large or complex parts, where it can be difficult to apply a torch or other heating method.

3.     Resistance preheating: This involves passing an electric current through the base metal to heat it prior to welding. Resistance preheating is often used for spot welding and other types of resistance welding.

4.     Furnace preheating: In this method, the base metal is heated in a furnace prior to welding. Furnace preheating is often used for large parts or when a high degree of precision is required.

        The specific type of preheating used will depend on the material being welded, the welding process being used, and the available equipment. In some cases, a combination of preheating methods may be used to achieve the desired preheat temperature and reduce the risk of cracking in the welded joint.

1.3  When Pre-heating required?

Preheating is required in welding when the base metal being welded is at risk of cracking or other defects due to the welding process. Preheating is typically necessary when welding materials that are thicker than 1 inch or that have a high carbon or alloy content.

Here are some situations where preheating may be required:

a)   Welding thick sections of steel: When welding thick sections of steel, preheating is often necessary to reduce the risk of cracking due to thermal stresses.

b)   Welding materials with high carbon or alloy content: Materials with high carbon or alloy content are more prone to cracking during welding, and preheating can help to reduce this risk.

c)    Welding in cold environments: When welding in cold environments, preheating can help to ensure that the base metal is at a suitable temperature for welding.

d)   Welding high-strength steels: High-strength steels are more prone to cracking during welding, and preheating can help to reduce this risk.

e)    Welding materials with low thermal conductivity: Materials with low thermal conductivity, such as cast iron, can benefit from preheating to ensure that the base metal is at a consistent temperature throughout the welding process.

Overall, preheating is required in welding when the base metal is at risk of cracking or other defects due to the welding process. The specific preheating requirements will depend on the material being welded, the welding process being used, and other factors such as ambient temperature and humidity.

 1.4    Advantages of Pre-heating?

                  i.     Reduced risk of cracking: Preheating can help to reduce the risk of cracking in the welded joint by reducing thermal stresses.

                 ii.     Improved weld quality: Preheating can help to produce a higher-quality weld by reducing porosity, increasing weld penetration, and producing a stronger and more reliable weld. 

               iii.     Reduced hydrogen cracking: Preheating can help to drive out any moisture or hydrogen that may be present in the base metal, reducing the risk of hydrogen cracking. 

               iv.     Better control over welding parameters: Preheating can help to ensure that the welding process is consistent and that the joint is at a consistent temperature throughout the welding process.

1.5    Metal Temperature Measuring While Preheating?

Measuring metal temperature is a critical step when preheating for welding. Here are a few ways to measure metal temperature during preheating:

1.    Infrared thermometer: An infrared thermometer, also known as a laser thermometer, can be used to measure the temperature of the metal surface from a distance. This is a non-contact method of measuring temperature and can provide a quick and accurate reading.

2.    Thermocouple: A thermocouple is a device that can be attached to the metal surface using a clamp or magnet. The thermocouple measures the temperature of the metal and sends the data to a digital readout device. This method provides an accurate and reliable measurement of the metal temperature.

3.    Tempilstik: A Tempilstik is a crayon-like device that is used to mark the metal surface. When the metal reaches the temperature required for preheating, the Tempilstik melts, indicating that the metal is at the correct temperature.

4.    Heat-sensitive paint: Heat-sensitive paint can be applied to the metal surface prior to preheating. The paint changes colour as the metal heats up, indicating the temperature of the metal surface.

It is important to note that the method used to measure metal temperature will depend on the specific requirements of the welding process and the material being welded. Additionally, it is essential to ensure that the temperature is measured accurately and consistently throughout the preheating process to ensure that the metal is at the correct temperature for welding.

1.6    Pre-heating Requirement As per ASME B31.3

            As per standard ASME B31.3, This preheat may apply for tack welds, repair welds, seal welds also. As per P-Numbers (P-Nos. and Group Nos. from ASME BPVC, Section IX, QW/QB-422) materials has different minimum temperature requirements. As per Engineering design and WPS it may vary and unlisted materials also shall be specified in WPS.

Crayons, thermocouple pyrometers are using for temperature indicator and measuring devices as per ASME B31.3.

From the point of welding for a distance of the larger of 75 mm (3 in.) or 1.5 times the greater nominal thickness. The base metal temperature for tack welds shall be at or above the specified minimum temperature for a distance not less than 25 mm in all directions from the point of welding.

 

                                                        Photo from book ASME B31.3 2020, Para 330.