Main Surface Treatment Processes and Industry Adaptation Specifications for Couplings

In mass production and machining of industrial couplings, bulk heat treatment determines the substrate strength and toughness of components, while surface treatment directly controls the wear resistance, dimensional accuracy, corrosion resistance and service life of parts. It serves as a core process to address common coupling failures including gear tooth wear, shaft journal scoring, in-service corrosion and mating surface failure. At present, mainstream surface treatments in the coupling industry fall into two major categories: surface hardening treatment and protective & decorative treatment. Each process has formed standardized industry specifications covering applicable scenarios, processing workflows and performance characteristics. Proper process selection can effectively improve the stability of transmission components and reduce equipment maintenance costs.

Nitriding is the most widely adopted surface hardening process for high-precision, heavy-load couplings. It is mainly applied to coupling parts with quenched and tempered substrates such as 40Cr, 42CrMo and 38CrMoAl, covering key friction positions including crowned gear tooth surfaces, transmission shaft journals and precision mating sleeves. This low-temperature process operates at 500–580 °C. Its primary advantage lies in minimal workpiece distortion, so the comprehensive mechanical properties of the substrate obtained from prior quenching and tempering will not be compromised. A dense, hard nitrided layer forms on the coupling surface after nitriding, delivering ultra-high hardness and excellent anti-seizure and anti-scuffing capacity. It can effectively prevent surface wear under high-speed operation and reciprocating sliding conditions, and provides mild inherent rust resistance. Due to its thin hardened layer, this process is suitable for high-precision, low-shock and continuous-friction transmission scenarios, and acts as the core strengthening process for mass production of precision couplings.

Carburizing and quenching is used for wear strengthening of specially structured couplings, primarily for cross shafts, special-shaped transmission teeth and other components made of low-carbon alloy steel such as 20CrMnTi. Unlike nitriding, carburizing is carried out at temperatures above 900 °C. Carbon atoms penetrate into the part surface layer, followed by bulk quenching and low-temperature tempering to form a thick high-hardness hardened layer. The key strength of this process is its deep carburized layer and strong resistance to impact wear. It withstands heavy loads and intermittent impact friction, making up for the weakness of thin nitrided layers which cannot endure severe impact. Nevertheless, high-temperature treatment causes certain machining distortion, so precision grinding is required afterwards to correct dimensional accuracy. It is only applicable to special coupling parts of low-carbon steel and cannot be used for conventional quenched-and-tempered alloy steel couplings.

Induction surface hardening (high-frequency hardening) is a cost-effective surface hardening process for general couplings. It is commonly applied to shaft journals and positioning mating surfaces of ordinary couplings made of quenched and tempered 45# steel and 40Cr. High-frequency current rapidly heats only the workpiece surface layer, which is then instantly quenched. Only the surface metallographic structure is hardened while the core retains the toughness and strength achieved by quenching and tempering. This process features moderate cost, high production efficiency and controllable hardened layer thickness. It meets wear resistance requirements for general medium-load applications without stringent precision demands, and is widely used for standard couplings in fans, water pumps and general mechanical equipment. It represents the most cost-effective surface hardening solution for civilian transmission parts.

Black oxide finishing (bluing) is a basic protective process for couplings with no hardening effect, only providing rust prevention and uniform appearance. It fits all carbon steel and alloy steel coupling finished parts. As a final-stage process, bluing must be performed after all heat treatment, machining and precision grinding operations are completed. Chemical oxidation generates a dense oxide film on the part surface to isolate air and moisture, preventing minor rust under ambient conditions and unifying surface color. This process does not alter workpiece dimensions or mechanical properties, and has low cost. It is a standard final finishing process across the industry, suitable only for normal dry working conditions and unable to withstand humid environments or corrosive media.

For couplings used in chemical, marine and highly humid corrosive environments, galvanizing and hard chromium plating are commonly adopted for corrosion protection in the industry. Electrogalvanizing delivers economical corrosion protection for ordinary coupling housings under light-load and humid conditions. Hard chromium plating offers both corrosion protection and wear resistance: its surface layer has high hardness and strong corrosion resistance. It is mostly used on precision coupling mating surfaces and piston-rod type transmission components in corrosive environments, resisting attack by acid, alkali and water vapor and greatly extending service life under harsh operating conditions.

The industry follows unified process guidelines. All coupling surface treatments abide by the core workflow: bulk quenching and tempering first, followed by surface hardening, and protective treatment last. Bulk heat treatment builds the substrate toughness and strength of parts; surface hardening solves wear and scoring issues; protective treatment provides rust and corrosion resistance. The three processes have separate and irreplaceable functions. Meanwhile, common defects such as reversed process sequence, excessive grinding of hardened layers and substrate property degradation caused by high-temperature processes must be strictly avoided. This standardized surface treatment system fully guarantees operational stability and durability of couplings under diverse working conditions.