The manufacturing of seamless stainless steel elbows involves multiple technical challenges, mainly reflected in three aspects: material properties, process control, and surface treatment.
Its material properties pose the following processing difficulties:
1. Severe work hardening
Austenitic stainless steel can harden several times as much as carbon steel during the cutting process, resulting in a sharp reduction in the service life of the tool when working in the hardened area. It is necessary to adopt a progressive cold bending process to reduce the amount of single deformation, and design a pre compensation mold to offset the rebound deviation (usually requiring an increase of 2 ° bending).
2. Cutting temperature and tool wear
Stainless steel has poor thermal conductivity (only one-third of carbon steel), and high temperatures are concentrated in the cutting edge area during cutting, accelerating tool wear. Special hard alloy cutting tools (such as drill bits with TiAlN coating) should be selected, and internal cooling cutting fluid should be used to reduce the edge temperature. The control line speed should be ≤ 60m/min, and the feed rate should be ≤ 0.15mm/r.
3. The chips are tough and easy to stick to the knife
Martensitic stainless steel chips are strong and tough, and are prone to friction with the front cutting surface, causing fusion welding and damaging surface smoothness. We need to optimize the chip breaker design and add extreme pressure cutting oil to reduce friction.
The challenges faced in the molding process are as follows:
1. Hot push molding control
Wall thickness uniformity: During intermediate frequency heating and pressing, temperature gradients cause fluctuations in the wall thickness reduction rate (usually requiring a reduction rate of ≤ 8%). It is necessary to maintain the strength of the mold through water cooling technology in the core mold holes and monitor the temperature field distribution in real time.
Surface oxidation: When heated above 800 ℃, oxide skin is generated, which needs to be pushed under inert gas protection.
2. Cold bending deformation
Thin walled elbows (wall thickness ≤ 3mm) are prone to cracking on the outer side and wrinkling on the inner side when bent with a small radius. It is necessary to use segmented and multi pass molding, combined with hydraulic servo correction system to compensate for deformation. The rebound amount is as high as 15 ° -20 °, and the compensation amount needs to be predicted through finite element simulation.
3. Defects in welding joints
During the welding of elbows and straight pipe sections, intergranular corrosion occurs in the heat affected zone. Laser arc composite welding is required to reduce heat input and restore corrosion resistance through post weld solution treatment.
In addition, there are also difficulties in surface quality and accuracy:
1. The inner wall roughness exceeds the standard (>0.8 μ m)
Electrolytic polishing ensures Ra ≤ 0.2 μ m.
2. Bending angle deviation (＞± 1 °)
Five axis CNC precision machining calibration.
3. Mirror polishing with micro scratches
Multi stage sand belt polishing (180 → 320 → 600 mesh).