First, what are the core process-related causes of cracking in precision steel pipes?
Precision steel pipes have extremely high requirements for dimensional accuracy, surface quality, and mechanical properties. Cracking is often directly related to subtle deviations in the manufacturing process. Based on industry production practices, the core process-related causes can be categorized into four main types:
(1) Defects in the Pre-treatment Process of Raw Materials for Precision Steel Pipes
1) Substandard Material Purity: Excessive levels of harmful impurities such as sulfur and phosphorus in the raw steel billet can easily form low-melting-point sulfides and phosphides during rolling, leading to weakened grain boundaries and intergranular cracking during subsequent processing or use.
2) Unresolved Internal Defects in the Billet: The steel billet may contain original defects such as shrinkage cavities, porosity, and inclusions. If these defects are not screened by flaw detection or subjected to high-temperature heat diffusion treatment during the pre-treatment stage, they will become stress concentration sources during subsequent forming and heat treatment, triggering cracking.
3) Incomplete surface cleaning: Oxide scale, rust, and scratches on the billet surface are not thoroughly removed through processes such as shot blasting and pickling. During rolling, these defects are “pressed” into the steel pipe surface, forming micro-cracks that expand into macroscopic cracks during subsequent processing or under stress.
(2) Deviations in the forming process parameters of precision steel pipes
A) Unreasonable rolling/drawing processes:
1) Improper deformation control: Excessive deformation in a single operation leads to excessive residual stress inside the steel pipe. If intermediate annealing is not performed in time to eliminate this stress, stress accumulation can cause cracking. Insufficient deformation can easily result in uneven microstructure and reduced toughness.
2) Uneven rolling temperature: Excessively high heating temperature during hot rolling leads to coarse grains and decreased toughness. Insufficient temperature results in insufficient material plasticity, increased deformation resistance, and a tendency to tensile cracking.
3) Insufficient die precision: Inadequate dimensional tolerances and surface finish of the rolls or drawing dies cause severe friction with the steel pipe surface, resulting in surface scratches or localized stress concentration, inducing cracking.
4) Defects in Bending/Straightening Processes: Excessive pressure, speed, or bending radius during straightening can lead to excessive localized plastic deformation in the steel pipe, resulting in residual tensile stress. This is particularly problematic in thin-walled precision steel pipes, easily causing longitudinal or transverse cracking.
(3) Improper Heat Treatment Processes for Precision Steel Pipes
A) Deviations in Heating and Cooling Parameters:
1) Excessively High Quenching Temperature: This results in coarse austenite grains, forming coarse martensite after quenching, increasing brittleness, and making the pipe prone to quenching cracks. Insufficient temperature leads to incomplete austenitization, leaving residual ferrite in the microstructure, resulting in a mismatch between strength and toughness, and subsequent cracking under stress.
2) Inappropriate Cooling Rate: Excessively rapid cooling creates a large temperature difference between the inside and outside of the steel pipe, generating significant thermal stress. Simultaneously, the stress from the martensitic transformation process is superimposed, triggering cracking. Insufficiently slow cooling fails to achieve the required high-strength microstructure and may result in unstable microstructures such as pearlite and bainite, reducing material reliability.
B) Missing or Incorrect Tempering Process:
1) No Tempering Treatment: Quenched steel pipes retain significant internal stress and have unstable microstructure, making them prone to delayed cracking during storage or initial use.
2) Tempering Temperature Too Low/Time Too Short: Internal stress is not fully eliminated, and the microstructure does not complete its stabilization transformation; excessively high tempering temperature leads to decreased strength, both potentially causing cracking.
3) Operational Issues During Heat Treatment: Excessively high local temperatures during heating and uneven medium distribution during cooling result in abnormal local microstructure and stress in the steel pipe, creating potential cracking hazards.
(4) Welding Process Defects in Precision Steel Pipes
1) Inappropriate Welding Parameters: Excessive welding current leads to coarse and brittle grains in the heat-affected zone; insufficient current results in incomplete penetration, forming defects such as slag inclusions and lack of fusion, becoming stress concentration points that crack after welding or under stress.
2) Incompatibility between welding materials and base metal: Significant differences in composition between the welding rod/wire and the base metal lead to heterogeneous structures at the weld joint, resulting in inconsistent mechanical properties and a high risk of cracking.
3) Improper post-weld treatment: Failure to perform post-weld tempering or stress-relieving annealing results in the superposition of residual welding stress and structural stress, or excessively rapid cooling after welding can cause cold cracking.
Second, Core Directions and Control Measures for Process Optimization of Precision Steel Pipes.
1) Raw material quality control: Select low-sulfur, high-purity steel billets; strengthen billet flaw detection to remove unqualified billets containing shrinkage cavities and inclusions; optimize surface cleaning processes to ensure thorough removal of defects such as oxide scale and scratches.
2) Optimization of forming process parameters: Develop reasonable deformation amounts based on the steel pipe material; incorporate intermediate annealing to eliminate residual stress; calibrate the accuracy of rolls and drawing dies; control rolling/drawing temperatures; optimize straightening pressure and speed to ensure the bending radius is ≥ 10-15 times the steel pipe diameter.
3) Standardized Heat Treatment Process: Precisely control quenching temperature, employing methods such as staged quenching and isothermal quenching to reduce cooling stress; Temper promptly after quenching, setting the tempering temperature according to performance requirements and ensuring sufficient tempering time; Use uniform heating equipment to ensure even distribution of the cooling medium and avoid localized stress concentration.
4) Standardized Welding Process: Select welding materials compatible with the base material, optimizing parameters such as welding current, voltage, and welding speed; Perform stress-relieving annealing promptly after welding to prevent cold cracking; Strengthen weld flaw detection to remove defects such as incomplete penetration and slag inclusions.
5) Process Quality Monitoring: Set up quality inspection points at key stages such as forming, heat treatment, and welding; Use methods such as hardness testing, metallographic analysis, and non-destructive testing to promptly detect abnormal microstructures and excessive stress, adjusting process parameters accordingly.
Post time: Jan-13-2026