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The reciprocating compressor plunger rod/piston rod is a key load-bearing component connecting the crosshead and the piston. It transmits the reciprocating force from the crankshaft to the piston and withstands alternating tensile and compressive stresses. Its performance directly affects the compressor's operating stability, sealing reliability, safety, and service life, making it one of the most critically loaded key parts in the piston assembly.
During operation, the piston rod moves back and forth inside the cylinder, transferring mechanical energy from the crankshaft to the gas and compressing it. Piston rods are usually made of high-quality forged alloy steel and have strict requirements for straightness, surface finish, and fatigue strength.
Piston rod material is selected differently according to the corrosiveness of the operating conditions, pressure rating, and medium characteristics. Under normal operating conditions, high-quality carbon steel or alloy steel can be selected. Under corrosive conditions, 17-4PH precipitation-hardening stainless steel conforming to NACE MR0175 has strong resistance to uniform corrosion and stress corrosion cracking, and is suitable for petrochemical, coal chemical, offshore platform, and other applications.
Our piston rods use the H1150M heat treatment process. H1150M features moderate strength, the best toughness, and optimal resistance to stress corrosion cracking, with smaller machining deformation and low residual stress, making it suitable for high-toughness, low-stress-corrosion-sensitive parts. For acidic oil and gas and operating conditions with hydrogen sulfide stress corrosion risk, it can be supplied in the ASTM A564 H1150M condition.
The working section of the piston rod that fits with the stuffing box and oil scraper ring must be surface hardened. Tungsten carbide (WC) spraying is a ceramic-like process with a high hardness of 60 HRC and excellent wear resistance. Lower surface roughness can reduce friction and wear with seals and extend packing service life.
17-4PH material must first undergo solution treatment before any other processing.
Heat the material to 1040°C (1900°F), hold, then air cool or oil cool to obtain a martensitic matrix, preparing for subsequent machining and age hardening.
After solution treatment, perform quenching and tempering, heat treatment, and other processes according to the material, drawings, and operating conditions.
High-temperature tempering is the key process for H1150M. Heat the material to 760°C, hold for 2 hours, and air cool. This is used to improve toughness and resistance to stress corrosion cracking. Then perform aging treatment to enable the material to reach the final strength, hardness, and toughness requirements.
All threads (screw threads) are rolled on a thread rolling machine, not formed by turning. Rolling can improve the fatigue strength at the thread root, surface density, and fiber continuity, making it more suitable for withstanding high-frequency alternating loads.
The working section of the piston rod is sprayed with a tungsten carbide (WC) coating. Hard chrome plating or nitriding can also be selected according to operating conditions. The coating thickness is controlled according to drawings and operating conditions.
Grind the piston rod according to drawing dimensions to reach the final dimensions. Precision grinding is used to ensure that roundness, cylindricity, coaxiality, straightness, and surface roughness meet drawing requirements.
Heat treatment inspection: heat treatment curve records and hardness testing. H1150M hardness is controlled at HRC 24~29.
Dimensions and geometric tolerances: inspect key dimensions, coaxiality, roundness, cylindricity, straightness, and total-length bending. The roundness and cylindricity of the working section reach IT6 grade, straightness ≤0.05 mm/m, and total-length bending ≤0.08 mm.
Surface quality: surface roughness Ra≤0.2 μm, coating thickness, coating adhesion, and appearance free of cracks and scratches.
Mechanical properties: measure tensile strength, yield strength, elongation, and impact toughness. Taking H1150M as an example: tensile strength Rm 790~860 MPa, yield strength Rp0.2 620~720 MPa, elongation ≥18%.
Non-destructive testing: perform ultrasonic testing (UT), magnetic particle testing (MT), penetrant testing (PT), etc., as required, to ensure no internal and surface defects.
Final inspection: full dimensional inspection according to drawings, visual inspection, and confirmation of marking and traceability information, ensuring product traceability and batch consistency.
