First, what are the causes of brittleness in the processing of high-carbon steel seamless pipes and fittings?
High-carbon steel, due to its high carbon content and large proportion of pearlite, is prone to brittle cracking during processing. The main contributing factors include:
A) Hardening and embrittlement of the microstructure: Failure to anneal promptly after rolling results in denser pearlite lamellars, leading to increased hardness (HRC > 35) and significantly reduced plasticity. Stress concentration during processing easily triggers cracks.
B) Accumulation of residual stress: 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 correction, in particular, easily generates microcracks.
C) Hydrogen-induced cracking: During welding or heat treatment, hydrogen penetrates the material and accumulates under stress, forming hydrogen white spots and reducing fracture toughness.
D) Inappropriate process parameters: Such as excessively high heat correction temperature (close to Ac3). Linear deformation can lead to coarse grains, or excessively rapid cooling can produce martensitic structures, exacerbating brittleness; excessive deformation during compaction straightening can exceed the material’s plasticity limit.
Second, what are the key technologies for controlling the brittleness of high-carbon steel seamless pipe fittings?
(1) Pretreatment Process Optimization
A) Stress-relief annealing: Anneal the fittings before processing at a temperature of 650-700℃ for 2-3 hours, then cool them in the furnace to below 300℃ to reduce residual rolling stress, homogenize the microstructure, control the hardness to HRC20-25, and improve plasticity.
B) Surface cleaning treatment: In addition to removing oil and oxide scale, shot blasting is used to remove surface microcracks. The bevel is preheated (150-200℃) before welding to reduce hydrogen adsorption.
C) Material pretreatment: For ultra-high carbon steel with a carbon content >0.8%, isothermal spheroidizing annealing can be used to convert pearlite into spherical pearlite, further improving plasticity and reducing processing brittleness.
(2) Brittleness Adaptation Adjustment of Straightening Process
1) Optimization of Mechanical Straightening Parameters:
A) Bulging Straightening: 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 due to excessive instantaneous stress. Straightening accuracy is allowed to be achieved in stages, reducing the amount of deformation per cycle.
B) Rolling Straightening: 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 reducing the rolling speed (3-5m/min) to reduce work hardening.
C) Precise Control of Hot Straightening Temperature: The heating temperature is strictly controlled 80-100℃ below the Ac3 line (the Ac3 line for high-carbon steel is approximately 720-780℃, and the actual heating temperature is…). (640-700℃) to avoid martensite formation after austenitization upon cooling; use medium-frequency induction heating to ensure uniform heating area, expanding the heating width to 1/2-2/3 of the pipe diameter to reduce local temperature gradients; use furnace cooling or slow cooling (covered with insulation cotton) during cooling, prohibiting air cooling or water cooling;
D) Composite straightening sequence adjustment: For high-carbon steel pipe fittings, adopt the process of “mechanical pre-straightening → stress-relief annealing → precision mechanical straightening”. First, eliminate some ellipticity (deviation ≤1mm) through mechanical straightening, and then perform precision straightening after annealing to release stress, avoiding the superposition of residual stress and straightening stress.
(3) Prevention and Control of Hydrogen-Induced Cracking
A) Hydrogen Control During Welding: Use low-hydrogen welding rods or wires, dry them before welding (hold the welding rods at 350-400℃ for 1 hour), maintain short arc operation during welding to reduce hydrogen intrusion; perform hydrogen removal treatment promptly after welding (hold at 200-250℃ for 2-4 hours);
B) Hydrogen Removal Treatment After Straightening: If the straightened pipe fittings involve welding processes, hydrogen removal treatment must be completed within 24 hours to avoid hydrogen accumulation; for thick-walled high-carbon steel pipe fittings (wall thickness ≥15mm), additional hydrogen removal annealing is required after straightening at 300-350℃ for 4 hours.
(4) Microstructure and Performance Control
After straightening, the pipe fittings undergo recrystallization annealing at 550-600℃ for 1.5-2 hours to eliminate work hardening and restore plasticity. For pipe fittings in critical operating conditions, quenching and tempering treatment (quenching + high-temperature tempering) 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, what are the effects of process optimization on high-carbon steel seamless pipe fittings?
Brittleness testing methods:
A) Mechanical property testing: Testing the hardness (controlled at HRC22-28), tensile strength (≥600MPa), and impact toughness (αk≥25J/cm²) of the corrected fittings to ensure toughness meets standards;
B) Non-destructive testing: Using ultrasonic testing (UT) to detect internal cracks and magnetic particle testing (MT) to detect surface and near-surface cracks, with a crack detection sensitivity ≥0.2mm;
C) Aging verification: Natural aging is extended to 72-120 hours. While testing ellipticity stability, impact tests are used to compare toughness changes before and after aging to ensure no aging brittleness.
D) Simulated working condition testing: For high-pressure fittings, hydrostatic tests (test pressure 1.5 times the working pressure) and fatigue tests (10⁶ cycles) are conducted to verify no risk of brittle fracture.
Fourth, what are the precautions for high-carbon steel seamless pipe fittings?
A) Dynamic parameter matching: Adjust process parameters according to the specific carbon content (0.6%-1.0%) and wall thickness (≤30mm) 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.
B) Process monitoring: 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 deviation.
C) Prohibition of unauthorized operations: Cold straightening of high-carbon steel pipe fittings is strictly prohibited (preheating to 100-150℃ is required when the ambient temperature is <10℃) to avoid low-temperature brittleness.
D) Enhanced quality traceability: Add microstructure performance test data and hydrogen content test results (using the hot extraction method, hydrogen content ≤2ml/100g) to the process record archive to achieve full-process traceability.
Post time: Jan-15-2026