What are the optimization schemes for the heat treatment process of precision steel pipe fittings

First, what are the optimization goals for precision steel pipe fittings?

(A) Performance Indicators: While ensuring dimensional accuracy (tolerance ≤ 0.1%), increase the tensile strength of carbon steel and alloy steel pipes by 20%-30%, optimize impact energy by more than 15%, control hardness uniformity within the range of HB±10, and eliminate defects such as temper brittleness and network cracks.

(B) Quality Stability: Reduce the heat treatment defect rate (deformation, cracking, and insufficient hardness) from the current level to below 1%, achieving consistent control in batch production.

(C) Production Efficiency: Through process optimization and intelligent control, reduce energy consumption by more than 25%, shorten the processing cycle by 10%-15%, and improve production turnover rate.

Second, how to optimize the pre-treatment process of precision steel pipe fittings?

Pre-treatment is the foundation for ensuring the heat treatment effect. It can effectively eliminate original stress and structural defects, laying the foundation for subsequent quenching and tempering processes.

(I) Pre-treatment before furnace loading

(A) Cold-drawn precision steel pipes.

Full annealing is preferred to eliminate work hardening. The heating temperature is controlled at 850-900℃, and the holding time is calculated at 2 hours per 25mm wall thickness. The pipes are then furnace cooled to room temperature to allow the pearlite structure to recrystallize, reducing hardness by 15-20 HB and increasing elongation to 8%-12%, facilitating uniform microstructure transformation during subsequent tempering.

(B) Forged steel pipes and fittings.

Pre-treatment must be completed within 24 hours of forging. Normalizing is used to refine the grains, eliminate forging stress, and prevent deformation and cracking due to stress concentration during tempering.

Before loading, the surface of the steel pipes and fittings is shot-blasted to remove oxide scale and oil, preventing surface decarburization during heating and ensuring uniform heating. During loading, the spacing between individual pieces should be ≥50mm to avoid uneven furnace temperature caused by overly dense stacking. Specialized fixtures are used to fix slender pieces, reducing the risk of subsequent deformation. (II) Furnace Preheating Optimization

A two-stage preheating process replaces the traditional single-stage preheating. The first stage preheats to 300-350℃ and holds for 1-1.5 hours, controlling the temperature difference between the inside and outside of the steel pipe fittings within 50℃. The second stage preheats to 600-650℃ and holds for 0.5-1 hour, further reducing the temperature gradient. This method effectively reduces thermal stress during heating, avoids localized overheating in thin-walled steel pipes, and shortens the austenitizing heating time, reducing energy consumption.

Third, How to Optimize the Core Tempering Process of Precision Steel Pipe Fittings?

By combining the hardenability and microstructure characteristics of different materials of precision steel pipes, the quenching and tempering temperatures, holding times, and cooling methods are precisely controlled to achieve a balance between strength and toughness.

(I) Quenching Process Optimization

(A) Austenitizing Temperature Control: Adjust the heating temperature according to the steel pipe material. For carbon steel, control the temperature at 850-870℃, and for alloy steel, control it at 860-880℃ to ensure complete dissolution of carbides and obtain uniform and fine austenitic grains. Use infrared thermography to monitor the pipe wall temperature in real time and implement closed-loop control of the heating power to keep the temperature error within ±5℃.

(B) Holding Time Optimization: Accurately calculate the holding time according to the material and wall thickness. The holding time is 0.6 min/mm for carbon steel and 0.8 min/mm for alloy steel. The minimum holding time is no less than 30 min, and the maximum is no more than 2 hours. This avoids excessively long holding times, leading to coarse grains, or insufficient holding times leading to incomplete microstructure transformation.

(C) Cooling Method Improvement: Replace the traditional single cooling medium with a staged quenching + nitrogen mist cooling composite process. For commonly used materials such as 45# and 40Cr, the material is first subjected to staged cooling in a 300-400℃ salt bath for 2-3 minutes to suppress the martensite transformation rate. Then, nitrogen mist cooling (cooling rate 30-50℃/s) is used to complete the subsequent cooling, ensuring that the bainite content reaches over 75% and the Vickers hardness is stabilized at 320-350 HV. Simultaneously, dimensional deformation is controlled within 0.05mm to avoid cracking caused by water quenching and uneven hardness caused by oil quenching.

(II) Tempering Process Optimization

(A) Precise Matching of Tempering Temperature: The appropriate tempering temperature should be selected based on the mechanical performance requirements of the steel pipes and fittings. Medium-temperature tempering (450-550℃) is suitable for applications requiring a balance between high strength and toughness, such as hydraulic cylinders. After holding at this temperature for 2 hours, the impact energy can increase from 25J to 45J, maintaining a tensile strength above 900MPa. High-temperature tempering (550-650℃) is suitable for steel pipes and fittings subjected to impact loads, such as gear shafts, effectively eliminating quenching stress and reducing hardness to HB220-250. For alloy steels prone to second-type tempering brittleness, the tempering temperature should avoid the sensitive range of 450-550℃, or oil cooling should be used after tempering to accelerate cooling and eliminate brittleness.

(B) Multi-stage tempering application: For heavy-duty, high-precision steel pipes and fittings, a "two-stage tempering" process is adopted. The first tempering temperature is 20-30℃ lower than the target temperature, held at that temperature for 1.5 hours, and then air-cooled to room temperature. The second tempering is performed at the target temperature for 2 hours, followed by air cooling. This process can further refine the carbide structure, improve hardness uniformity, reduce residual stress, and increase the bending fatigue life of steel pipes and fittings by more than 3 times.

(C) Furnace temperature uniformity control: A controlled atmosphere furnace with an air circulation fan is used for tempering. The furnace temperature uniformity is calibrated regularly (error ≤ ±3℃) to avoid local hardness deviations in steel pipes and fittings due to uneven furnace temperature. For mass production, zoned temperature control technology is used to adjust the heating power of each zone in real time to ensure a stable temperature field within the furnace.

Fourth, Quality Control and Defect Prevention System for Precision Steel Pipes and Fittings

(I) Process Quality Inspection

Establish a process parameter traceability system, collecting over 200 key parameters such as furnace temperature distribution, heating time, and cooling rate. Construct a heat treatment expert database to achieve full-process traceability for each batch of steel pipes and fittings, improving process stability by over 40%.

Randomly select 3-5 steel pipes and fittings from each batch for hardness testing (Brinell/Vickers hardness tester), tensile strength, and impact energy. Observe the microstructure using a metallographic microscope to ensure that the bainite and martensite ratio meets design requirements and the grain size reaches ASTM Grade 10 standard.

Use a coordinate measuring machine to inspect key dimensions of steel pipes and fittings, recording dimensional changes before and after heat treatment, and adjusting process parameters promptly to ensure tolerances meet standards.

(II) Common Defect Prevention

(A) Uneven/High/Low Hardness: This can be resolved by calibrating the furnace temperature, optimizing the furnace loading density, and adjusting the holding time. For high hardness, the tempering temperature can be appropriately increased; for low hardness, check the quenching temperature and cooling rate, and re-quench and temper if necessary.

(B) Deformation Cracking: A multi-stage preheating and graded cooling process is employed, along with specialized tooling and fixtures. Deformed pipe fittings undergo multiple straightening and heating treatments to eliminate deformation stress. Excessive heating is avoided to prevent large temperature differences between the surface and core, thus preventing network cracking.

(C) Temper Brittleness: Tempering temperature and cooling method are precisely controlled. Second-type temper brittleness can be eliminated by re-tempering and accelerated cooling. First-type temper brittleness requires re-quenching and adjustment of the tempering temperature.

(D) Surface Decarburization: A controlled atmosphere furnace is used for heating, with inert gas protection. The pipe fitting surface is thoroughly cleaned before loading into the furnace. If necessary, a plasma nitriding + low-temperature tempering composite process is used to increase the surface hardness to 800 HV while ensuring core toughness.