Flame Straightening Techniques in Metalworking

In the thermal straightening of metal materials, the most widely used is flame straightening with oxy-acetylene flame. Flame straightening is not only used in the preparation work of materials, but also can be used to correct the deformation of structures during the manufacturing process. Due to the convenience, flexibility, and low cost of flame straightening, its application is relatively widespread.

Metal materials have the physical properties of thermal expansion and contraction. When locally heated, the heated part of the material expands, but due to the low temperature of the surrounding material, the expansion is hindered. At this time, the heated metal is under compressive stress. When the heating temperature is between 600-700℃, the compressive stress exceeds the yield strength of the material at that temperature, resulting in compressive plastic deformation.

After stopping heating, the metal cools and contracts, resulting in the metal fibers in the heated area being shorter than before, creating new deformation. Flame straightening utilizes the new deformation caused by local heating of the metal to correct the original deformation. Therefore, understanding the deformation patterns caused by local heating of the flame is crucial to mastering flame straightening.

Figure 1 shows the deformation of steel plate, angle steel, and T-shaped steel during and after heating. The triangle in Figure 1 represents the heating area. As the metal fibers in the heated area contract upon cooling, the shaped steel bends towards the heating side.

a), b) Steel plate
c) Angle steel
d) T-shaped steel

During flame straightening, the deformation caused by heating must be in the opposite direction of the original deformation to offset and correct it. The heat source for flame straightening heating is usually oxy-acetylene flame, because it has high temperature and fast heating speed.

I. Flame Straightening Operation Methods

Flame straightening is a manual operation. To achieve better straightening results, it is necessary to control the heating location, time, and temperature of the flame according to the deformation condition of the workpiece. Different heating positions can correct deformations in different directions. The heating position should be chosen at the part with longer metal fibers, namely, the outer side of the material where bending deformation occurs.

Besides, the shape of the heating area on the heated workpiece significantly affects the correction direction and amount of deformation. The direction with the greatest difference in fiber length passing through the heating area is the direction with the greatest bending deformation of the workpiece. The amount of deformation is proportional to the length difference passing through the heating zone. Using flames with different heat levels can achieve varying correction capabilities.

If the flame heat is insufficient, the heating time will be prolonged, expanding the heated area and reducing the deformation difference between parallel fibers, making it difficult to flatten the deformation. Therefore, the faster and more concentrated the heating, the stronger the correction ability and the greater the amount of correction deformation.

For flame straightening of low-carbon steel and ordinary low-alloy steel, a heating temperature of 600-800℃ is commonly used. Generally, the heating temperature should not exceed 850℃ to avoid overheating the metal. However, the heating temperature should not be too low either, as it would lead to poor correction efficiency. The heating temperature can be roughly judged by the color of the steel surface when heated in production, with its accuracy depending on experience, as shown in Table 1.

Table 1 Steel Surface Color and Corresponding Temperature (Observed in Dark)

There are three ways of heating on the surface of deformed workpieces: point heating, line heating, and triangular heating.

Point heating refers to heating an area of a certain diameter in a round-shaped spot. The shape and number of hotspots are determined based on the deformation condition of the steel. Multi-point heating commonly uses a plum blossom pattern (see Figure 2a), and the diameter d of each point should be suitably larger for thick plates and smaller for thin plates, generally not less than 15 mm.

a) Point heating
b) Line heating
c) Triangular heating

The greater the deformation, the smaller the distance a between points should be, generally 50-100 mm.

During heating, when the flame moves in a straight line direction or simultaneously swings in a certain lateral direction in width, it is called line heating. There are three types: straight-through heating, chain heating, and belt heating (see Figure 2b). The transverse shrinkage of the heating line is generally greater than the longitudinal shrinkage, and the shrinkage amount increases as the width of the heating line increases, with the heating line width generally being 0.5-2 times the thickness of the steel. Line heating is usually used for structures with significant deformation.

When the heating area is triangular, it is called triangular heating (see Figure 2c). Because the heating area is large, the shrinkage amount is also large, and due to the uneven heating width along the height direction of the triangle, the shrinkage amount is also uneven, resulting in great bending deformation correction, often used for correcting bending deformation of rigid and significantly deformed components.

Table 2 shows the methods for acetylene flame straightening of common steel materials.

Table 2 Acetylene Flame Straightening Methods for Common Steel Materials

Blank material	Original deformation	Heating method	Sketch	Explanation
Thin steel plate (thickness not exceeding 8 mm)	Central bulge	Point heating		With the bulge facing up, clamp with a Kamaten. Hotspotsspaced 50-100 mm apart; use smaller value for larger deformation.Hot spot diameter ≥ 15mm, take the maximum plate thickness. If the deformation area is large, take more heating points. See the figure for heating sequence, supplemented by hammering.
		Linear heating		Clamp the bulged part facing up on the platform. The heating line trajectory includesThree types: straight line, wave line, and spiral line. The latter twoHave widths of (0.5~2) times the plate thickness. Longitudinal shrinkage along the heating line is less than transverse shrinkage. When there is a large amount of deformation,You can increase the line width and reduce the line spacing.
	One side is wavy	Linear heating		With the bulged part facing up, clamp three sides which are not deformed, heat both sides of the bulged part first, then surround towards the bulged part, and repeat heating if necessary.
Thick steel plate	Arched bending	Linear heating		Put on the platform, heat the highest part to 600~800°C, heating depth should not exceed 1/3 of the plate thickness, can repeat heating if necessary.
Steel pipe	Bending	Spot heating		Heat the convex surface (single or multiple rows of spots), moving quickly from spot to spot, heat row by row.The speed from spot to spot should be quick, heat one row at a time.
T-shaped steel	Side bending	Triangle heating		Heat the bulged part of the horizontal plate
				Heat the bulged part of the vertical plate
Angle steel	External bending	Triangle heating		Heat the raised part
I-beam	Side bending	Triangle heating		Heat the raised part
Channel steel	Local side bending	Linear heating		Two welding torches simultaneously performing wavy heating
Steel cylinder	Local curvature is too large	Linear heating		Heat along the generating line
	Local curvature is too small

II. Precautions for flame straightening operation

Pay attention to the following points during flame straightening operation:

1) Heating speed should be fast, heat should be concentrated, and minimize the heated area outside the heating zone, which can improve the straightening effect and achieve a larger amount of local shrinkage.

When correcting a large deformation area, whether using multiple points or multiple lines of heating, the heating areas must not overlap, otherwise, the material of the workpiece will be damaged. Before correction, points and lines for heating and their directions should be marked according to the size and degree of the deformation area. In one batch of heating, all points and lines should be evenly distributed, symmetrical, and staggered.

The entire heating process must be carried out in batches. When one batch meets the straightening requirements, no further heating is needed. Unplanned straightening processes are prohibited as they can ensure the effect of straightening and avoid overlapping of heated areas.

The forward and backward order of the heating points and lines in each batch must start from the edge of the deformation area. Excessive concentrated heating in the middle of the deformation area is prohibited, as it will cause excessive deformation in the area and make subsequent straightening difficult due to the material properties of that area.

2) In practical correction work, it’s common to use water to quickly cool the heated area after heating to accelerate metal shrinkage and improve correction efficiency. Compared to pure flame straightening, the efficiency can be more than tripled. This method is known as the water and fire straightening method.

The water and fire straightening method has certain limitations. When correcting low-carbon steel plates with a thickness of 2mm, the heating temperature should generally not exceed 600°C, and the distance between the water and fire should be closer.

When correcting steel plates with a thickness of 4~6mm, the heating temperature should be 600~800°C, and the distance between the water and fire should be 25~30mm. When correcting steel plates thicker than 8mm, water cooling is generally not considered due to the large stress caused by water cooling. For steel plates with hardening tendency (such as ordinary low alloy steel plates), the distance between water and fire should be greater.

For materials with high hardening tendencies (such as medium and high carbon steels or alloy steels), water fire correction methods cannot be used, and only a certain degree of air cooling can be performed to enhance deformation. When bending and correcting steel plates, the heating depth should be controlled within 1/4 to 1/3 of the plate thickness and should not be too deep, otherwise, it will greatly affect the effect of flame correction.

Although flame correction is a method with significant corrective effects, it is still relatively poor in controlling deformation amounts, especially for workpieces particularly sensitive to flame correction, such as the straightening correction of slender parts and the flattening correction of thin plates.

Therefore, for the correction of such workpieces with large deformation amounts, flame correction can only be used as a rough correction method, complemented by subsequent mechanical correction; for the correction of such workpieces with small deformation amounts and high requirements, flame correction should not (is prohibited) be used, otherwise, it will lead to new or even greater deformation.

3) To accelerate the contraction of the heating area, hammering is sometimes supplemented, but a wooden or copper hammer must be used, not an iron hammer.