Technical differences between cold pushing and hot pressing elbow forming processes

Cold pushing and hot pressing are the two mainstream forming processes for elbows made of stainless steel, carbon steel, and other materials. There are significant technical differences between the two in terms of temperature conditions, forming principles, equipment requirements, and product performance, which are suitable for the production of elbows with different specifications, materials, and working conditions. The following is a systematic comparison from the dimensions of core process characteristics, technical parameters, and applicable scenarios.

1、 Differences in Core Forming Principles and Temperature Conditions

This is the most fundamental difference between the two processes, which directly determines the subsequent processing flow and product characteristics.

Cold push elbow forming process Cold push forming is to apply axial thrust to the straight tube blank with the help of one-to-one die and thrust equipment at room temperature, so that the tube blank will undergo plastic deformation along the die cavity, thus bending to a specific angle of elbow. The core principle is to utilize the cold plasticity of metal, and through the guiding effect of the mold, gradually conform the pipe blank to the contour of the mold under thrust, forming the curvature and angle of the elbow. The entire process does not require heating of the billet, and the metal remains at room temperature.

The hot pressing forming process of hot pressed elbows requires heating the straight pipe blank to a temperature above the metal recrystallization temperature (generally 900 ℃~1100 ℃ for carbon steel and 1050 ℃~1200 ℃ for stainless steel), so that the metal material is in a hot state with good plasticity. Subsequently, the hot tube blank is pressed into the prefabricated elbow mold cavity using a hydraulic press or a pressure machine, and the shape of the mold is used to force plastic deformation of the tube blank. After cooling, the desired elbow size is obtained. Its core is to use high temperature to reduce the deformation resistance of metals and achieve large deformation bending forming.

2、 Differences in technical requirements between equipment and molds

The difference in process principles results in vastly different requirements for production equipment and molds between the two.

Characteristics of Cold Push Forming Equipment and Molds

Equipment requirements: A hydraulic thrust machine with appropriate precision is required, with stable thrust and controllable stroke to protect the uniform deformation of the pipe blank; Partial precision is suitable for elbow production, and a CNC system is needed to carefully adjust the thrust size and speed. The core of the equipment is to provide sufficient axial thrust to overcome the deformation resistance of the metal in a cold state.

Mold requirements: The mold has high precision, including key components such as core rods and outer molds, and the curvature radius and angle tolerance of the mold cavity need to be strictly controlled. The mold material should be made of high-strength wear-resistant (based on actual reports) steel (such as Cr12MoV), because the friction between the metal and the mold is high during cold forming, and the mold is prone to wear, requiring good performance and hardness. In addition, the surface smoothness of the mold is required to prevent scratching the surface of the tube blank.

Characteristics of equipment and molds for hot pressing forming

Equipment requirements: In addition to the hydraulic press, a one-to-one heating furnace (such as an intermediate frequency induction heating furnace) should also be equipped to achieve suitable and uniform heating of the pipe blank at a suitable speed; The production of some large elbows requires supporting material racks and transfer devices for the transportation of hot pipe blanks. The core requirement of hydraulic press is to provide sufficient radial pressure to press the hot tube blank into shape, and the stability requirement for pressure is lower than that of cold pushing process.

Mold requirements: The mold must have high temperature resistance (based on actual reports), commonly made of heat-resistant alloys or cast steel materials, and able to withstand the thermal impact of high-temperature pipe blanks; The precision requirements for the mold cavity are slightly lower than those for cold push molds, as cooling after hot metal deformation will result in definite shrinkage, which can be compensated by reserving shrinkage allowance. The mold needs to be equipped with exhaust holes to prevent the air inside the cavity from being unable to be discharged during the pressing process, which may cause bubbles or depressions on the surface of the elbow.

3、 Differences in Processing Techniques and Processes

The production processes and key procedures of the two processes are significantly different, directly affecting production efficiency and product qualification rate.

The process flow of cold pushing forming includes raw material inspection → straight pipe billet cutting → billet end face processing (groove treatment) → mold installation and debugging → billet insertion into the core rod → axial thrust forming → demolding → adjustment and correction → surface treatment (rust removal, polishing) → non-destructive testing → finished product. The key process lies in thrust control and adjustment correction: cold formed elbows are prone to springback, and it is necessary to carefully adjust the thrust size and mold parameters to reduce the amount of springback; For elbows with high precision requirements, secondary adjustments are required after forming to confirm that the angle and curvature radius meet the standards.

The process flow of hot pressing forming includes raw material inspection → cutting of straight pipe blank → heating of pipe blank (with meticulous temperature control) → placing the hot pipe blank into the mold → hydraulic pressing → pressure holding cooling → demolding → heat treatment (stress relief annealing) → machining (groove, end face) → non-destructive testing → finished product. The key process lies in temperature control and heat treatment: excessive heating temperature can cause coarse metal grains and reduce the mechanical properties of the elbow; If the temperature is too low, the deformation resistance is high and cracks are prone to occur. After compression, there is a large amount of residual thermal stress inside the elbow, which needs to be relieved by stress annealing treatment to prevent cracking during subsequent use.

4、 Product performance and quality differences

The process characteristics directly determine the mechanical properties, dimensional accuracy, and surface quality of the elbow.

Performance characteristics of cold push elbow

Dimensional accuracy: Good precision, curvature radius and angle tolerance can be controlled within ± 0.5 °, good wall thickness uniformity, suitable for producing small-diameter, thin-walled, and precision appropriate elbows (such as DN15~DN300, wall thickness ≤ 10mm).

Mechanical properties: Cold forming can cause work hardening of the metal, resulting in an increase in the strength and hardness of the elbow, but a slight decrease in plasticity; If the elbow is used for high temperature or high stress conditions, subsequent annealing treatment is required to eliminate work hardening.

Surface quality: The surface is smooth, free of oxidation skin, and does not require complex surface cleaning processes. It is suitable for producing products with high requirements for surface aesthetics (such as decorative and grade pipeline elbows).

Performance characteristics of hot pressed elbows

Dimensional accuracy: The accuracy is relatively low, with angle tolerances generally ranging from ± 1 ° to ± 2 °. The uniformity of wall thickness is slightly poor, especially for large-diameter elbows, which are prone to local thinning or thickening of wall thickness; Suitable for producing large-diameter, thick walled elbows (such as DN300 or above, wall thickness ≥ 10mm).

Mechanical properties: After high-temperature forming, the metal grains are uniformly refined. After stress relief annealing, the comprehensive mechanical properties of the elbow are excellent, with good toughness and impact resistance. It is suitable for harsh working conditions such as high pressure and high temperature (such as high-pressure pipelines in petrochemical industry and steam pipelines in power plants).

Surface quality: There is oxide scale on the surface, which needs to be removed by acid washing, sandblasting, etc. The surface smoothness is lower than that of cold push elbows.

5、 Differences in applicable scenarios

The applicability of the two processes is highly compatible with the specifications, materials, and working conditions of the elbow.

Applicable scenarios for cold pushing forming

Material: Suitable for stainless steel (304, 316), low carbon steel, etc. with good plasticity.

Specification: Small caliber, thin wall, suitable precision for elbows.

Working conditions: pipelines with low pressure, normal temperature, high requirements for dimensional accuracy and surface quality, such as food processing, pharmaceuticals, water supply and drainage industries.

Applicable scenarios for hot pressing forming

Material: Suitable for carbon steel, alloy steel, thick walled stainless steel, etc., especially for high-strength alloy materials that are difficult to deform.

Specification: Large caliber, thick walled, large curvature radius elbow.

Working conditions: harsh working conditions with high pressure, high temperature, and high stress, such as high-pressure oil and gas transportation in petrochemicals, steam pipelines in power plant boilers, and pipelines in mining machinery.

6、 Production efficiency and cost difference

Cold pushing forming: Long mold debugging time, short single forming cycle, suitable for mass production of small-diameter elbows; The equipment investment is high, the mold cost is expensive, but there are few subsequent processing steps, and the overall cost is moderate.

Hot pressing forming: The mold debugging is simple, and the single forming cycle is long (including heating and cooling time), suitable for mass production of large-diameter elbows; The equipment investment (heating furnace+hydraulic press) is relatively high, and the raw material loss is slightly large, but the requirement for mold accuracy is low, and the mold cost is relatively low.