Key Manufacturing Processes of Quenched and Tempered Piston Rods
The key manufacturing processes for quenched and tempered piston rods encompass six major steps: material selection, pre-treatment, quenching and tempering heat treatment, machining, surface treatment, and quality inspection. Details are as follows:
1. Material selection
Based on the working conditions of the piston rod (e.g., load, environmental corrosion), medium-carbon steel (e.g., 45# steel), alloy structural steel (e.g., 40Cr, 35CrMo), or stainless steel (e.g., 304, 316) are prioritized. These materials must exhibit high strength, high toughness, and corrosion resistance. Non-destructive testing (e.g., ultrasonic inspection) is used to ensure the absence of internal defects such as cracks or inclusions.

2. Pre-treatment
Forged blanks undergo annealing or normalizing to eliminate forging stress and improve machinability. For example:
45# steel: Normalizing (heated to 840–860°C, held, then air-cooled).
40Cr steel: Annealing (heated to 850°C, held, then furnace-cooled).

3. Quenching and tempering heat treatment
Quenching:
The piston rod is heated above its critical temperature (e.g., 840–860°C for 45# steel, 850°C for 40Cr steel), held for a specific duration, and rapidly cooled (e.g., oil or water quenching) to form a martensitic structure.
Tempering:
Immediately after quenching, high-temperature tempering (e.g., 550–650°C for 45# steel, 600–650°C for 40Cr steel) is performed, followed by air cooling. This produces a tempered sorbitic structure, balancing strength, hardness, plasticity, and toughness.

4. Machining
Rough Machining:
Turning and milling are used to remove excess material and preliminarily shape the outer diameter, threads, and other features.
Precision Machining:
Grinding and roller burnishing improve dimensional accuracy and coaxiality. Roller burnishing creates a work-hardened layer, enhancing wear resistance and reducing surface roughness (to Ra ≤ 0.2 μm).
Straightening:
Post-heat treatment deformation is corrected through straightening to ensure straightness (generally ≤ 0.15 mm).

5. Surface treatment
Chrome Plating:
A hard chromium layer (0.03–0.05 mm thick) is electroplated onto the surface to improve corrosion resistance and surface hardness (up to HV 800–1000). Post-plating grinding or polishing achieves a surface roughness of Ra 0.2–0.4 μm.
Nitriding:
Gas or ion nitriding forms a nitride layer (50–100 μm deep) on the surface, increasing hardness (up to HV 1000–1200) and wear resistance while maintaining core toughness.
Thermal Spraying:
Techniques like high-velocity oxygen fuel (HVOF) spraying apply tungsten carbide (WC) or ceramic coatings to further enhance wear and corrosion resistance.

6. Quality inspection
Dimensional Accuracy:
Coordinate measuring machines (CMM) and external micrometers are used to inspect dimensional accuracy, coaxiality, cylindricity, and other geometric tolerances.
Hardness Testing:
Hardness testers verify surface and core hardness to ensure compliance with design requirements.
Non-Destructive Testing (NDT):
Ultrasonic and magnetic particle testing detect internal and surface defects such as cracks or inclusions.
Surface Roughness Measurement:
Surface roughness testers ensure the surface finish meets operational requirements.