What is the situation where the size of stainless steel seamless elbows becomes unstable after cold drawing?
The dimensional instability of stainless steel seamless elbows after cold drawing process is mainly related to four factors: material, process, mold, and stress. Let’s first talk about the manifestations of dimensional instability: fluctuations in outer diameter, uneven wall thickness, bending deformation, etc.
1. Magnification of material defects themselves
Segregation and inclusions: Steel pipe billets exhibit sulfur phosphorus segregation or central looseness (such as grade B 2.5 segregation). During cold drawing, the low melting point impurity accumulation zone preferentially deforms, resulting in a wall thickness difference of over ± 0.3mm, and in severe cases, longitudinal cracks may appear. If titanium containing precipitates aggregate (such as pale yellow Ti precipitates), it will hinder metal flow and cause local depressions.
Residual banded structure: The hot-rolled billet was not fully homogenized and annealed, and the grains were elongated along the deformation direction after cold drawing. The deformation resistance of grains with different orientations varied greatly, resulting in a cross-sectional ellipticity exceeding 0.12mm.
The original size exceeds the standard: the outer diameter tolerance of the tube blank is greater than ± 0.3mm or the uneven wall thickness is greater than 10%, and the error is amplified by 3-5 times after cold drawing.
2. Improper control of cold drawing process
Incomplete annealing: for example, insufficient temperature (below 900 ℃), resulting in incomplete work hardening, poor material plasticity, large fluctuations in springback during forced drawing, and outer diameter deviation of ± 0.1mm; For example, if the cooling is too slow, carbides will precipitate at grain boundaries, and uneven local hardness will cause deformation and loss of control.
Lubrication failure: Insufficient viscosity or uneven coating of lubricant, friction coefficient>0.1, resulting in surface scratches and stress concentration points, and a 15% reduction in wall thickness at curved areas; Uneven metal flow resistance, with curvature radius fluctuations exceeding ± 5% for elbows.
Mismatching of drawing parameters: For example, when the deformation is too large and the compression rate of a single pass is greater than 25%, the material flow is turbulent, and the inner wall thickness of the elbow increases while the outer wall decreases; For example, if the speed is too fast, the accumulation of frictional heat causes the mold temperature to rise by 100 ℃, and thermal expansion leads to a reduction of the outer diameter by 0.05~0.1mm.
3. The precision of the mold system is out of control
Mold wear: Mold hole enlargement>0.02mm, elbow outer diameter exceeding tolerance+0.15mm.
Core rod positioning deviation: wall thickness difference>0.3mm, elbow concentricity failure.
Mold design defect: When the entrance cone angle is less than 8 °, the metal flow is obstructed and the surface has bamboo knots.
4. Residual stress release deformation
Cold drawing stress not eliminated: Failure to timely relieve stress annealing (300 ℃ × 1h) after drawing, resulting in elastic recovery (TC4 titanium alloy elbow rebound of 0.3%~0.5%, curvature radius exceeding tolerance) or aging deformation (austenitic stainless steel elbow length shrinkage of 0.2mm/m after 7 days of storage or processing).
Subsequent processing induced release: Local heating during welding or cutting (>200 ℃) causes residual stress to redistribute, resulting in a bend angle deviation of>2 °.
What is the situation where the size of stainless steel seamless elbows becomes unstable after cold drawing?
The dimensional instability of stainless steel seamless elbows after cold drawing process is mainly related to four factors: material, process, mold, and stress. Let’s first talk about the manifestations of dimensional instability: fluctuations in outer diameter, uneven wall thickness, bending deformation, etc.
1. Magnification of material defects themselves
Segregation and inclusions: Steel pipe billets exhibit sulfur phosphorus segregation or central looseness (such as grade B 2.5 segregation). During cold drawing, the low melting point impurity accumulation zone preferentially deforms, resulting in a wall thickness difference of over ± 0.3mm, and in severe cases, longitudinal cracks may appear. If titanium containing precipitates aggregate (such as pale yellow Ti precipitates), it will hinder metal flow and cause local depressions.
Residual banded structure: The hot-rolled billet was not fully homogenized and annealed, and the grains were elongated along the deformation direction after cold drawing. The deformation resistance of grains with different orientations varied greatly, resulting in a cross-sectional ellipticity exceeding 0.12mm.
The original size exceeds the standard: the outer diameter tolerance of the tube blank is greater than ± 0.3mm or the uneven wall thickness is greater than 10%, and the error is amplified by 3-5 times after cold drawing.
2. Improper control of cold drawing process
Incomplete annealing: for example, insufficient temperature (below 900 ℃), resulting in incomplete work hardening, poor material plasticity, large fluctuations in springback during forced drawing, and outer diameter deviation of ± 0.1mm; For example, if the cooling is too slow, carbides will precipitate at grain boundaries, and uneven local hardness will cause deformation and loss of control.
Lubrication failure: Insufficient viscosity or uneven coating of lubricant, friction coefficient>0.1, resulting in surface scratches and stress concentration points, and a 15% reduction in wall thickness at curved areas; Uneven metal flow resistance, with curvature radius fluctuations exceeding ± 5% for elbows.
Mismatching of drawing parameters: For example, when the deformation is too large and the compression rate of a single pass is greater than 25%, the material flow is turbulent, and the inner wall thickness of the elbow increases while the outer wall decreases; For example, if the speed is too fast, the accumulation of frictional heat causes the mold temperature to rise by 100 ℃, and thermal expansion leads to a reduction of the outer diameter by 0.05~0.1mm.
3. The precision of the mold system is out of control
Mold wear: Mold hole enlargement>0.02mm, elbow outer diameter exceeding tolerance+0.15mm.
Core rod positioning deviation: wall thickness difference>0.3mm, elbow concentricity failure.
Mold design defect: When the entrance cone angle is less than 8 °, the metal flow is obstructed and the surface has bamboo knots.
4. Residual stress release deformation
Cold drawing stress not eliminated: Failure to timely relieve stress annealing (300 ℃ × 1h) after drawing, resulting in elastic recovery (TC4 titanium alloy elbow rebound of 0.3%~0.5%, curvature radius exceeding tolerance) or aging deformation (austenitic stainless steel elbow length shrinkage of 0.2mm/m after 7 days of storage or processing).
Subsequent processing induced release: Local heating during welding or cutting (>200 ℃) causes residual stress to redistribute, resulting in a bend angle deviation of>2 °.