First, why is preheating necessary for steel pipe welding?
1. Preheating for steel pipe welding prevents cold cracking: Welding is a process of rapid localized heating and cooling. Too rapid cooling can easily lead to the formation of hard and brittle martensite in the weld and heat-affected zone, similar to quenching steel by plunging it into cold water. Simultaneously, hydrogen in the weld doesn't have time to escape and is trapped inside. Excessive hydrogen, combined with the tension from welding stress, causes cracks to appear subtly, sometimes invisible immediately after welding, but appearing overnight or even several days later, which is particularly troublesome. Preheating slows down the cooling rate, allowing hydrogen time to diffuse and reducing martensite formation, thus reducing the risk of cold cracking at its source.
2. Preheating for steel pipe welding reduces welding stress: During welding, the weld area is heated to a very high temperature, while the surrounding base material remains cool. This uneven thermal expansion and contraction leaves significant residual stress inside after welding. Excessive stress can lead to deformation or even direct cracking. Preheating is like "warming up" the workpiece first, reducing the temperature difference between the weld and the base metal, making the temperature field more uniform, and naturally reducing the internal stress after welding.
3. Preheating for steel pipe welding improves joint performance: Some materials (such as high-strength steel, heat-resistant steel, and cast iron) are particularly sensitive to cooling rates. If the weld cools too quickly, the joint becomes hard and brittle with poor toughness. After preheating, the cooling process becomes more gradual, allowing the weld and heat-affected zone to form a more ideal microstructure, resulting in better toughness, stronger crack resistance, and significantly improved overall joint performance.
4. Preheating for steel pipe welding removes moisture and reduces hydrogen sources: This point is often overlooked, but it is actually very important. If there is water, moisture, rust, or condensation on the surface of the base metal or on both sides of the bevel, the moisture will decompose during welding, producing hydrogen. Hydrogen is one of the main culprits of cold cracking and porosity. Even for thin plates that don't require preheating to prevent cracking, it's recommended to lightly heat them with a flame before welding in the following situations: high ambient humidity (such as the rainy season in southern China), outdoor storage of the plates, dew or rain streaks on the surface, oil stains near the bevel, rust absorbing moisture, and the temperature doesn't need to be very high—just warm and dry to the touch (approximately 50-80℃). This simple action can remove most of the moisture, greatly reducing the risk of porosity and delayed cracking.

Second, three commonly used preheating methods for steel pipe welding in production are discussed.
In practice, the main methods used are flame heating, resistance heating, and induction heating. Infrared heating and whole-furnace heating are either impractical or have too demanding conditions.
1. Flame heating for steel pipe welding preheating: The most traditional and flexible method: directly heating the workpiece with an oxygen, acetylene, propane, or natural gas flame. This is the oldest method and the most commonly used on-site.
Advantages: Simple equipment—just a heating torch and a gas cylinder are enough; low cost; easy to move; suitable for outdoor operations.
Disadvantages: Temperature relies entirely on the operator's experience, making precise control difficult; heating is uneven, with one side often hot while the other cools; the heating speed is also relatively slow.
Besides simple handheld torches, flame bars (fire chains) are frequently used in production for preheating large workpieces before welding. A row of flame outlets, drilled from steel pipes, forms a continuous flame curtain when connected to a gas source. Placing it on both sides of the weld seam creates a uniform heating band in one pass, several times more efficient than a single torch. It is particularly suitable for preheating large-diameter cylindrical circumferential seams and large structural components. When welding cylindrical circumferential seams, the flame bar is placed at the bottom for heating, while submerged arc welding is done at the top, maintaining a continuous temperature for the base material.
Several points to note when making and using flame bars: The nozzle spacing should be uniform (100~200mm), and the flame length should be roughly consistent; maintain an appropriate distance (100~200mm) between the flame bar and the workpiece surface during use, moving it back and forth or pausing briefly is acceptable; the key is to ensure each nozzle is unobstructed and the flame is stable, avoiding localized overheating or flameout.
2. Resistance Heating for Steel Pipe Welding Preheating: Precise Temperature Control, Stable Quality: Ceramic resistance heating elements (also called tracked heaters) are attached to both sides of the weld, covered with insulation cotton. When energized, the resistance wire heats up, and the heat is conducted to the workpiece. Combined with thermocouples and a temperature controller, a temperature curve can be set for automatic heating and heat preservation.
Advantages: Very accurate temperature control, relatively uniform heating, and automatic temperature control. Programmable operation according to process requirements ensures quality.
Disadvantages: The heating element must be tightly attached to the workpiece surface, making placement difficult in complex shapes or confined spaces; the heating element is prone to damage over time.
Applications: Pressure vessels, pipelines, alloy steel components, and other plates ≥50mm thick, as well as important structures; suitable for long-term heat preservation or important welds requiring strict temperature control.
3. Induction Heating for Steel Pipe Welding Preheating: Fast, Clean, and Highly Efficient: An induction coil is placed around the workpiece, and a medium-frequency current is applied, causing the workpiece to heat up internally through electromagnetic induction. This is considered a more advanced method.
Advantages: Very fast heating speed and high efficiency; precise temperature control and good repeatability; no open flame, clean working environment.
Disadvantages: Higher equipment cost. For regular shapes like pipes, flexible cable coils are already quite flexible; however, for complex and irregular workpieces, coil arrangement is not as convenient as flame heating, and strong magnetic fields interfere with infrared guns, requiring thermocouples for temperature measurement.
Applications: Mass production, automatic pipe welding, on-site welding of long-distance oil and gas pipelines, and applications requiring high heating speed and cleanliness.
Two minor questions about induction heating:
(1) Medium frequency or high frequency? Medium frequency (1~10 kHz) is the mainstream for welding preheating. Medium frequency has a moderate penetration depth (several to tens of millimeters), heating thoroughly without only heating the surface. Low frequency is too slow, and high frequency is too shallow (only suitable for thin plates or surface hardening). Medium frequency should be chosen when purchasing equipment.
(2) How thick a workpiece can it heat? Medium frequency induction heating is best suited for steel plates or pipe walls of 10~50mm. For thicknesses less than 10mm, reduce power to prevent overheating; for thicknesses of 50-80mm, heating is possible but requires extended time and monitoring of the inner temperature; for thicknesses exceeding 80mm, the effect decreases significantly, and resistance heating or low-frequency induction heating is recommended.

Third, which materials should be preheated before welding? 
Generally, materials with high carbon content/carbon equivalent and high hardenability are considered to require preheating to prevent cold cracking. The following materials should be preheated before welding:
Medium carbon steel and high carbon steel (carbon content > 0.30%): 45, 50 steel, and tool steel, etc.
Low-alloy high-strength steel (such as Q355, Q460, Q690, etc.): Preheating is generally recommended when the plate thickness exceeds 20mm or the carbon equivalent > 0.45%.
Heat-resistant steel (such as Cr-Mo steel, 12Cr1MoV, 15CrMo, etc.).
Cast iron: Poor weldability, generally requiring high-temperature preheating (300-600℃).
Thick plates (usually >20mm): Even if the material itself is not sensitive, increased plate thickness will accelerate cooling; preheating is recommended.

Fourth, do stainless steel and duplex stainless steel require preheating before welding?
(1) Austenitic stainless steel (e.g., 304, 316L): Generally, preheating is not required; in fact, the interpass temperature should be controlled to prevent it from becoming too high.
Reason: Austenitic stainless steel has poor thermal conductivity and a high coefficient of thermal expansion. Preheating will prolong the high-temperature dwell time, increasing the risk of intergranular corrosion (i.e., chromium carbide precipitation at grain boundaries, leading to decreased corrosion resistance). Furthermore, austenitic stainless steel itself does not have a tendency to cold crack, so preheating does not help prevent cracking.
In practice, it is sufficient to ensure the bevel is dry and free of oil and water before welding. The interpass temperature is usually controlled below 150℃, and some grades even require it to be below 100℃. If the ambient temperature is very low (e.g., outdoors in winter), you can lightly brush it with a flame to remove condensation, but do not heat it. 
(2) Duplex Stainless Steel (e.g., 2205, 2507)
Special Cases: Small, thin parts do not require preheating; thick plates or those with high restraint require low-temperature preheating.
The microstructure of duplex stainless steel consists of ferrite and austenite. Rapid cooling during welding leads to excessive ferrite and insufficient austenite, affecting corrosion resistance and toughness. However, excessively high preheating temperatures promote the precipitation of harmful phases.
The usual practice is: For plates less than 10mm thick and with low restraint, preheating is not necessary; simply control the interpass temperature to not exceed 100~150℃.
For thicker plates (e.g., greater than 15mm) or with strong structural restraint, low-temperature preheating, generally 50~100℃, can be used to reduce the cooling rate and prevent excessive ferrite formation in the weld and heat-affected zone.
Most importantly, strictly control the heat input and interpass temperature; avoid using high current for rapid heating.

Summary
(1) Purpose of Preheating for Steel Pipe Welding: For carbon steel and low-alloy steel, to prevent cracking, reduce stress, and improve joint performance. 
(2) Medium carbon steel, high-strength steel, heat-resistant steel, and cast iron must be preheated; ordinary low-carbon steel depends on plate thickness and environment; austenitic stainless steel does not require preheating; thin duplex steel does not require preheating, while thick duplex steel requires low-temperature preheating.
(3) Regarding heating methods, small parts and ordinary components use flame heating, thick plates and important structures use resistance heating, and mass production uses induction heating. Remember: temperature according to the process, width must be sufficient, temperature measurement must be accurate, and interpass temperature must not drop.
(4) Welding preheating may seem like an insignificant process, but these seemingly minor steps often determine whether the weld will be usable.