The question of when exactly preheating of welded structures is required and what temperature to choose for this is one of the most critical in industrial production and auto repair. Errors in calculations can lead to the formation of cracks, deformations or complete destruction of the unit under load, which is unacceptable for critical elements of a car or special equipment. The correct thermal regime allows you to minimize residual stresses and ensure high quality welds.
The need for heat treatment before starting work is determined by a complex of factors, among which the chemical composition of the material, its thickness and environmental conditions dominate. Ignoring these parameters often becomes the cause of defective products. In this article we will analyze in detail the physical processes occurring in the metal when heated, and algorithms for calculating optimal values for various alloys.
There is a misconception that metal should always be heated if it seems “cold”, or vice versa - never if the steel is “good”. Reality is dictated by strict physical and chemical laws. Preheating - this is not just a formality, but a technological necessity dictated by the properties of the crystal lattice and the cooling rate of the fusion zone.
Influence of chemical composition and carbon equivalent
The first and most important parameter that determines the need for heating is the chemical formula of the alloy. The key indicator here is carbon equivalent (Ceq), which is calculated based on the percentage of alloying elements: manganese, chromium, molybdenum, vanadium and others. The higher this indicator, the worse the weldability of the metal without additional thermal preparation.
The high content of carbon and alloying additives increases the tendency of steel to harden in the heat-affected zone. With rapid cooling, hard and brittle structures such as martensite are formed in this zone. They become the centers for the occurrence of cold cracks. For low-carbon steels (where C < 0.25%), preheating is often not required if the thickness of the parts is small and the air temperature is positive.
Formula for calculating carbon equivalent
To calculate Ceq, the formula is used: Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. If the result exceeds 0.45%, the risk of cracking increases sharply, and heating becomes mandatory.
However, for alloy and high-carbon steels the situation changes dramatically. Here preheat temperature can reach 200-300°C and higher. The exact value depends on the specific steel grade specified in GOST or international standards (for example, AWS D1.1). Engineers always rely on material data sheets or spectral analysis results.
- 🔥 Low carbon steels (St3, St37): heating is usually not required for thicknesses up to 30 mm.
- ⚙️ Medium carbon steels (St45, 40X): heating to 150-250°C is required depending on the thickness.
- 🛡️ High-alloy and tool steels: require strict temperature control up to 350°C and above.
The higher the carbon equivalent in the steel, the higher the preheat temperature must be to prevent hardening structures.
Dependence of temperature on the thickness of welded parts
Metal thickness is the second critical factor that directly affects the rate of heat removal from the welding zone. The massive parts act like a giant radiator, instantly removing heat from the weld pool. This leads to a sharp drop in temperature and, as a consequence, an increase in the cooling rate, which is equivalent to hardening.
There is a direct correlation: the thicker the wall or sheet, the higher it should be preheat temperature. For thin sheets (up to 10-15 mm), the heat dissipation is insignificant, and the metal cools more slowly relative to its volume. At the same time, when working with thick-walled pipes or truck frames (>30-40 mm), it is almost impossible to obtain a high-quality weld even on low-carbon steels without heating.
Tables of standards often provide threshold thickness values, the excess of which dictates the mandatory use of heat treatment. For example, for steel with a carbon equivalent of 0.4%, heating to 100°C may be required already with a thickness of 20 mm, and for the same steel with a thickness of 50 mm the temperature will increase to 175°C.
It is important to consider that when multilayer welding of thick elements, the interpass heating temperature must also be maintained at a certain level. The product should not be allowed to cool completely between passes, as this will cause the same stress problems as no initial warm-up.
Influence of ambient temperature and humidity
Environmental conditions are often underestimated, but they play a significant role in the thermodynamics of the welding process. Low air temperature increases the temperature gradient between the welding zone and the surrounding area, accelerating cooling. Therefore, winter welding of even ordinary structural steels often requires forced heating.
Moisture is the hidden enemy of seam quality. When heated, moisture contained in the air or on the surface of a metal breaks down into hydrogen and oxygen. Atomic hydrogen actively penetrates into the molten metal, causing hydrogen cracking. Preheating promotes the evaporation of moisture from the surface and reduces the likelihood of this defect.
⚠️ Attention: At air temperatures below +5°C and relative humidity above 90%, welding work without special shelter (warmhouse) and heating is prohibited for most critical structures. The risk of defects approaches 100%.
There are special correction factors for calculating the heating temperature depending on climatic conditions. If the standard temperature for a given steel is 100°C, then when working in cold weather (-10°C and below) it must be increased by 25-50°C. It is also critical to remove snow, ice and frost from edges before starting any work.
- ❄️ At temperatures below 0°C, heating is required for all thicknesses over 15 mm.
- 💧 High humidity requires increased exposure time at drying temperature.
- 🌬️ Strong winds accelerate cooling, necessitating rising temperatures or the use of screens.
The role of structural rigidity and stress levels
Joint stiffness is a parameter that describes the ability of a structure to resist deformation during welding. The more rigid the assembly (for example, a T-joint with two walls or a closed loop), the higher the occurrence of residual stresses. The metal cannot contract and expand freely, which leads to ruptures in the weld area.
For structures with a high degree of rigidity (High Restraint), the preheating requirements are much stricter. Heating reduces the yield strength of the metal in the weld area, making it more ductile. This allows the metal to “stretch”, compensating for shrinkage, without cracking. In such cases, the temperature may be raised above standard recommendations.
Particular attention should be paid to welding in confined spaces or when placing seams where elements intersect. Here the stress concentration is maximum. Usage local heating only in the junction area may be insufficient; Sometimes it is necessary to warm up the entire part to equalize the temperature field.
To determine the stiffness of an assembly, estimate the number of directions in which the metal can deform. If movement is limited on all sides, use the maximum heating mode from the table for a given steel grade.
Engineers often use special calculation methods or software simulations to estimate stress levels in complex assemblies. However, in practice, in a car service or workshop, they are guided by empirical rules: if the part is massive and rigidly fixed, it is necessary to heat it, even if the thickness of the metal is average.
Table of approximate preheat temperatures
To quickly make decisions, experts use lookup tables. Below are averaged data for the most common steel grades used in mechanical engineering and equipment repair. These values are basic and can be adjusted depending on specific conditions.
| Steel grade / Ceq | Thickness (mm) | Ambient temperature (°C) | Recommended heating temperature (°C) |
|---|---|---|---|
| St3 (Ceq < 0.35) | up to 30 | > 0 | Not required |
| St3 (Ceq < 0.35) | 30 - 50 | < 0 | 100 - 120 |
| 09G2S (Ceq ~ 0.45) | 10 - 20 | > 0 | 100 - 120 |
| 09G2S (Ceq ~ 0.45) | > 20 | Any | 150 - 175 |
| 40X / 45 (Ceq > 0.55) | Any | Any | 200 - 300 |
It is important to understand that the data in the table is for reference only. For critical structures such as suspension components, frames or pressure vessels, the requirements of the design documentation or work plan (WPS) must be strictly followed. Self-indulgence in choosing the temperature is unacceptable.
⚠️ Attention: Exceeding the heating temperature above 300-350°C for carbon steels can lead to overheating and deterioration of the mechanical properties of the metal (coarse grain). Observe the upper limit!
Control methods and heating methods
Once the required temperature is determined, the issue of measuring and maintaining it becomes critical. Visual assessment (by eye or by heat color) is extremely unreliable and is not allowed in professional welding. For monitoring, contact thermocouples, pyrometers or special temperature indicator pencils.
Thermal indicator pencils (temperature crayons) melt when a certain temperature is reached, leaving a greasy mark. This is a simple and cheap method of monitoring in the field. For more accurate control, electronic thermometers with thermocouples are used, which allow you to monitor the temperature in real time.
☑️ Warm-up control
Heating methods are selected depending on the conditions and scope of work. Workshops often use gas burners (propane-oxygen or propane-air) due to their mobility. For large volumes and precise control, induction heaters or electric ovens are used. The main requirement is uniform heating throughout the entire welding zone, usually at a distance of 75-100 mm from the cutting edges on each side.
It must be remembered that heating must be carried out smoothly to avoid the creation of new thermal stresses. Sudden local heating of cold metal can itself provoke a crack. After completion of welding, some grades of steel require slow cooling (insulation with asbestos materials or placement in an oven) to relieve residual stresses.
What is interpass temperature?
This is the minimum temperature that the metal must be at before applying the next layer of weld. It is usually equal to or slightly higher than the preheat temperature. Violation of this parameter negates the whole point of the initial warm-up.
Is it necessary to heat aluminum before welding?
Aluminum has high thermal conductivity, so for thick parts (>10-15 mm) heating to 100-150°C is desirable to improve penetration. However, heating aluminum alloys above 200°C is dangerous - they lose strength. It is also important to remove the oxide film.
Is it possible to use a hair dryer for heating?
An ordinary construction hair dryer will not provide the required temperature (maximum 600°C at the outlet, but on metal it will be much less) and heating power for thick metal. For welding purposes, only gas torches or specialized electric heaters are used.
How long should the temperature be maintained after heating?
The holding time depends on the thickness of the metal. The general rule is: at least 1 minute for every 25 mm of thickness, but at least 5 minutes to ensure the entire section is heated and moisture is removed. It is important that not only the internal mass of the metal warms up.
Is it dangerous to overheat above the recommended temperature?
Yes, it's dangerous. For many steels there is an upper limit (usually 250-300°C), exceeding which leads to temper brittleness or grain growth, which reduces the toughness of the weld. The critical threshold for most structural steels is 400°C - above this value, the structure of the metal begins to change irreversibly.
Is preheating required when using TIG welding?
Yes, the welding method (MIG/MAG, TIG, MMA) affects the heat input, but does not change the physical properties of the metal. If the steel is prone to hardening or the part is thick, heating is required regardless of whether you cook with argon or an electrode. TIG often produces a lower heat input, so the risk of underheating is even higher.