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Views: 0 Author: Site Editor Publish Time: 2026-09-21 Origin: Site
In a reciprocating compressor, the piston rod is the core transmission component connecting the piston and the crosshead. It undertakes multiple functions such as power transmission, motion guidance, sealing fit, and load bearing. 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. Especially in petrochemical, coal chemical, and offshore platform applications involving corrosive media such as H₂S, CO₂, and acidic water vapor, the material selection, heat treatment process, surface strengthening, and quality inspection of the piston rod are particularly important. Taking the H1150M condition 17-4PH precipitation-hardening stainless steel piston rod as an example, this article systematically introduces its product positioning, material selection, heat treatment process, manufacturing process, and inspection requirements.
As a key transmission part in the piston assembly of a reciprocating compressor, the piston rod first needs a clear functional positioning. It connects the piston and the crosshead and mainly performs the following functions:
Power transmission: Transmits the reciprocating force output by the crosshead to the piston, driving the piston to reciprocate in the cylinder to achieve gas intake, compression, and discharge.
Sealing fit surface: The rod body fits closely with the stuffing box and oil scraper ring, preventing the compressed medium from leaking out of the cylinder while blocking crankcase lubricating oil from entering the compression chamber, thereby ensuring medium purity.
Motion guidance: Cooperates with the guide ring and stuffing box to constrain the piston motion trajectory and reduce piston eccentric wear and abnormal cylinder wear.
Load bearing: Withstands alternating compressive stress, frictional shear force, and gas impact force throughout the entire process, and must have sufficient fatigue strength and impact toughness.
These functions determine that the piston rod is not a single load-bearing part, but a key component that simultaneously affects transmission, sealing, guidance, and service life. Therefore, subsequent structural design, material selection, and manufacturing processes must all focus on high strength, high toughness, high precision, and high reliability.
The structural design of the piston rod revolves around power transmission, guidance, and sealing fit. It usually adopts an integral high-strength round shaft structure, manufactured by integral forging or from high-quality bar stock. One end is connected to the piston head through fine threads, a shoulder clamp, or a taper connection, while the other end is connected to the crosshead or connecting rod mechanism through threads, a flange, or a hydraulic tightening connection. The use of fine threads and large-radius transitions can minimize stress concentration and avoid fatigue fracture under alternating loads.
Critical datum surfaces are established in a single setup, combined with multi-process precision machining and straightening, with strict control of straightness, roundness, and coaxiality, ensuring that the piston rod maintains stable axial precision during high-speed reciprocating motion and reducing the risk of off-center loading and abnormal packing wear.
From a structural perspective, the design focus of the piston rod is to reduce stress concentration, ensure reliable connection, and maintain motion precision. Once the structure is determined, material selection becomes the key link that determines its ability to adapt to operating conditions.
Material selection is carried out differently according to the corrosiveness of the operating conditions, pressure rating, and medium characteristics, and is mainly divided into three categories:
Materials for normal operating conditions: High-quality carbon steel and alloy steel, such as 45#, 40Cr, 42CrMo, 38CrMoAlA, etc. After quenching and tempering + surface hardening, they have high strength, high toughness, and good wear resistance, and are suitable for general scenarios with non-corrosive or weakly corrosive media such as air, nitrogen, and natural gas, with working pressure ≤20 MPa.
Materials for corrosive operating conditions: For corrosive media containing H₂S, CO₂, acidic water vapor, etc., 17-4PH precipitation-hardening stainless steel, 2Cr13 martensitic stainless steel, and 316L austenitic stainless steel conforming to NACE MR0175 are selected. They have strong resistance to uniform corrosion and stress corrosion cracking, and are suitable for acidic operating conditions in petrochemical, coal chemical, offshore platform, and other applications.
Materials for special operating conditions: For hydrogen compression scenarios, Inconel718 nickel-based alloy and stable austenitic stainless steel with excellent hydrogen embrittlement resistance are preferred; for special scenarios requiring lightweight, non-magnetic, and strong corrosion resistance, carbon fiber reinforced composite piston rods can also be used.
Overall, there is no universal solution for material selection; the key lies in matching the operating conditions. For scenarios with high corrosion risk such as acidic oil and gas, 17-4PH precipitation-hardening stainless steel combined with a suitable heat treatment condition is an important solution that balances strength, toughness, and resistance to stress corrosion cracking.
For acidic oil and gas and operating conditions with hydrogen sulfide stress corrosion risk, 17-4PH precipitation-hardening stainless steel can be used and supplied in the ASTM A564 H1150M condition. H1150M features moderate strength, the best toughness, and optimal resistance to stress corrosion cracking, with smaller machining deformation and low residual stress.
Tensile strength Rm: 790~860 MPa
Yield strength Rp0.2: 620~720 MPa
Elongation: ≥18%
Hardness: HRC 24~29
Condition | Heat Treatment Route | Hardness HRC | Core Application |
|---|---|---|---|
H1150 | Solution treatment → 621℃×4h single aging | 28~37 | General machinery, balancing strength and toughness |
H1150D | Solution treatment → 621℃×4h air cooling → again 621℃×4h air cooling (double isothermal aging) | 27~33 | NACE acidic oil and gas conditions |
H1150M | Solution treatment → 760℃×2h → 621℃×4h (modified double-temperature) | 24~29 | High toughness, low stress corrosion sensitivity, lowest hardness, best impact |
The heat treatment route of H1150M can be simply recorded as: solution treatment → 760℃×2h air cooling → 621℃×4h air cooling. Among these, the 760℃ high-temperature pre-tempering is the core step that distinguishes H1150M from H1150/H1150D, and 621℃ is the precipitation aging.
The working section that fits with the stuffing box and oil scraper ring must undergo surface hardening treatment. The mainstream processes include:
Hard chrome plating: thickness 0.05~0.25 mm;
Nitriding: suitable for 38CrMoAlA material;
Tungsten carbide (WC) spraying: a ceramic-like process with hardness up to 60 HRC and optimal wear resistance.
After tungsten carbide spraying, the surface roughness is Ra≤0.2 μm, which can reduce friction and wear with seals and extend packing service life.
Piston Rod with Tungsten carbide (WC) spraying
From raw material to finished product, the machining of the piston rod needs to follow a clear sequence: first complete the material solution treatment, then perform quenching and tempering and heat treatment, followed by thread rolling, surface spraying, and precision grinding. Each process affects the final performance, so process control is particularly important.
17-4PH material must first undergo solution treatment before other processing can be carried out. Solution treatment Condition A: hold at 1040℃ (1900°F), air cool/oil cool to obtain a martensitic matrix, preparing for subsequent machining and age hardening.
After solution treatment, quenching and tempering, heat treatment, and other processes are carried out according to the material, drawings, and operating conditions. Taking H1150M as an example, the heat treatment route defined by ASTM A564 is implemented:
First stage: 760℃ (1400°F) hold for 2h, air cool;
Second stage: 621℃ (1150°F) hold for 4h, air cool.
Conventional alloy steel adopts quenching and tempering + surface hardening; 38CrMoAlA can adopt nitriding treatment.
All threads (screw threads) are rolled on a thread rolling machine, not formed by turning. Rolling can improve thread root fatigue strength, 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, with hardness up to 60 HRC and optimal wear resistance. Hard chrome plating or nitriding can also be selected according to operating conditions. The coating thickness is controlled according to drawings and operating conditions.
After tungsten carbide (WC) spraying on the surface, the piston rod is ground according to drawing dimensions to reach the final dimensions. Precision grinding is adopted, and the roundness and cylindricity tolerances of the rod body working section reach IT6 grade, straightness deviation ≤0.05 mm/m, and total length bending ≤0.08 mm, ensuring uniform sealing fit and avoiding leakage or abnormal wear.
Material inspection: Chemical composition, mechanical properties, material certificate, and metallographic examination when necessary.
Heat treatment inspection: Heat treatment curve records and hardness testing. H1150M hardness is controlled at HRC 24~29; H1150 is 28~37 HRC; H1150D is approximately 27~33 HRC.
Dimensions and geometric tolerances: 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: Tensile strength, yield strength, elongation, and impact toughness. Taking H1150M as an example: Rm 790~860 MPa, Rp0.2 620~720 MPa, elongation ≥18%.
Non-destructive testing: 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.
The H1150M heat-treated 17-4PH stainless steel piston rod, through reasonable material selection, standardized heat treatment, tungsten carbide surface strengthening, thread rolling, and precision grinding, can maintain stable strength, toughness, and sealing fit performance under harsh conditions such as acidic oil and gas and hydrogen sulfide stress corrosion risk. Combined with full-process quality control from material, heat treatment, dimensions, and surface to non-destructive testing, it can provide reliable assurance for the safe, efficient, and long-term operation of reciprocating compressors.
