What are the methods for brittleness control and process optimization in the machining of high-carbon steel seamless pipe fittings

First, the core causes of brittleness in the machining of high-carbon steel seamless pipe fittings.

High-carbon steel, due to its high carbon content and large proportion of pearlite, is prone to brittle cracking during machining. The main contributing factors include:

1. Hardening and embrittlement of the seamless steel pipe fitting's microstructure: Failure to anneal promptly after rolling results in denser pearlite lamellars, leading to increased hardness and significantly reduced plasticity. Stress concentration during machining easily triggers cracks.

2. Accumulation of residual stress in the seamless steel pipe fitting: During processes such as cutting, welding, and ellipticity correction, localized plastic deformation and temperature gradients lead to the superposition of internal stresses. Excessive pressure during mechanical straightening, in particular, easily generates microcracks.

3. Hydrogen-induced cracking in seamless steel pipe fittings: During welding or heat treatment, hydrogen gas penetrates the material's interior, accumulating under stress to form hydrogen white spots, reducing fracture toughness.

4. Improper process parameters for seamless steel pipe fittings: For example, excessively high hot straightening temperatures lead to coarse grains, or excessively rapid cooling rates produce martensitic structures, exacerbating brittleness; excessive deformation during rolling straightening exceeds the material's plastic limit.

Second, Key Technologies for Brittleness Control of Seamless Steel Pipe Fittings

(I) Optimization of Pretreatment Process for Seamless Steel Pipe Fittings

1. Stress-Relief Annealing of Seamless Steel Pipe Fittings: Before processing, seamless steel pipe fittings are annealed at a temperature controlled at 650-700℃ for 2-3 hours, then cooled in the furnace to below 300℃ to reduce residual rolling stress, homogenize the microstructure, control the hardness to HRC20-25, and improve plasticity.

2. Surface Cleaning Treatment of Seamless Steel Pipe Fittings: In addition to removing oil and oxide scale, shot blasting is used to remove surface microcracks. The bevel is preheated before welding to reduce hydrogen adsorption.

3. Material Pretreatment of Seamless Steel Pipe Fittings: For ultra-high carbon steel with a carbon content >0.8%, isothermal spheroidizing annealing can be used to transform pearlite into spherical pearlite, further improving plasticity and reducing processing brittleness.

(II) Brittleness Adaptation Adjustment of Straightening Process for Seamless Steel Pipe Fittings

1. Optimization of Mechanical Straightening Parameters for Seamless Steel Pipe Fittings:

- Expansion Straightening of Seamless Steel Pipe Fittings: Pressure is gradually increased, with each pressure increase not exceeding 10% of the material's yield strength. The holding time is extended to 30-60 seconds to avoid cracking caused by excessive instantaneous stress. Straightening accuracy is allowed to be achieved in stages, reducing the amount of deformation per cycle.

- Rolling Straightening of Seamless Steel Pipe Fittings: Roller pressure is reduced to 50%-60% of the material's yield strength, with a single rolling amount of ≤0.3mm. The number of rolling cycles is increased, adopting a "small deformation, multiple cycles" mode, while simultaneously reducing the rolling speed to minimize work hardening.

2. Precise Control of Thermal Straightening Temperature for Seamless Steel Pipe Fittings: The heating temperature is strictly controlled 80-100℃ below the Ac3 line to avoid the formation of martensite after austenitization upon cooling. Medium-frequency induction heating is used to ensure a uniform heating area, and the heating width is expanded to [missing information - likely a percentage] of the seamless steel pipe fitting's diameter. 1/2-2/3, reduce local temperature gradient; use furnace cooling or slow cooling during cooling, prohibiting air cooling or water cooling;

3. Adjustment of composite straightening sequence for seamless steel pipe fittings: For high-carbon steel seamless steel pipe fittings, adopt the process of "mechanical pre-straightening → stress-relief annealing → precision mechanical straightening". First, eliminate part of the ellipticity through mechanical straightening, release stress through annealing, and then perform precision straightening to avoid the superposition of residual stress and straightening stress.

(III) Prevention and control of hydrogen-induced cracking in seamless steel pipe fittings

- Hydrogen control during welding: Use low-hydrogen welding rods or wires, dry them before welding, maintain short arc operation during welding to reduce hydrogen intrusion; perform hydrogen removal treatment promptly after welding.

- Dehydrogenation treatment after straightening: If the straightened seamless steel pipe fitting involves welding, hydrogen removal treatment must be completed within 24 hours to avoid hydrogen accumulation; for thick-walled high-carbon steel seamless steel pipe fittings, additional dehydrogenation annealing is added after straightening, at a temperature of 300-350℃, for 4 hours.

(IV) Microstructure and Property Control of Seamless Steel Pipe Fittings

After straightening, the seamless steel pipe fittings undergo recrystallization annealing at 550-600℃ for 1.5-2 hours to eliminate work hardening and restore plasticity. For seamless steel pipe fittings operating under critical conditions, quenching and tempering treatment can be used to obtain tempered sorbite microstructure, balancing strength and toughness. The quenching temperature is 820-850℃, and the tempering temperature is 550-600℃, ensuring an impact toughness αk ≥ 30 J/cm².

Third, Verification of the Process Optimization Effect of Seamless Steel Pipe Fittings

1. Mechanical Property Testing of Seamless Steel Pipe Fittings: The hardness, tensile strength, and impact toughness of the corrected seamless steel pipe fittings were tested to ensure that toughness meets standards.

2. Non-destructive Testing of Seamless Steel Pipe Fittings: Ultrasonic testing was used to detect internal cracks, and magnetic particle testing was used to detect surface and near-surface cracks, with a crack detection sensitivity of≥0.2mm.

3. Aging Verification of Seamless Steel Pipe Fittings: Natural aging was extended to 72-120 hours. While testing ellipticity stability, impact tests were conducted to compare toughness changes before and after aging, ensuring no aging brittleness.

4. Simulated Working Condition Testing of Seamless Steel Pipe Fittings: Hydrostatic tests and fatigue tests were performed on seamless steel pipe fittings under high-pressure conditions to verify the absence of brittle fracture risk.

Fourth, Precautions and Long-Term Protection for Seamless Steel Pipe Fittings

1. Dynamic Matching of Seamless Steel Pipe Fitting Parameters: Adjust process parameters according to the specific carbon content and wall thickness of the high-carbon steel. The higher the carbon content, the smaller the deformation should be, and the annealing temperature needs to be appropriately increased.

2. Process Monitoring of Seamless Steel Pipe Fittings: Use an infrared thermometer to monitor the temperature in real time during the straightening process, and a pressure sensor to provide feedback on the straightening pressure to avoid parameter deviations.

3. Prohibited Operations for Seamless Steel Pipe Fittings: Cold straightening of high-carbon steel seamless steel pipe fittings is strictly prohibited to avoid low-temperature brittleness.

4. Enhanced Quality Traceability of Seamless Steel Pipe Fittings: Add microstructure and performance test data and hydrogen content test results to the process record archive to achieve full-process traceability.