In industrial production and engineering applications, enhancing material surface hardness is crucial, catering to core needs like wear resistance, fatigue strength, corrosion resistance, and high temperature oxidation resistance. This article summarizes four mainstream methods to help readers gain a clear overview for precise selection in practice.
I. Surface Quenching Technology
Surface quenching is a core method for surface hardening via rapid heating and cooling, with common processes including induction hardening, flame hardening, and laser or electron beam hardening. These technologies induce surface phase transformation to form hardened phases without altering chemical composition, requiring materials to have a certain carbon content and good hardenability.
Induction hardening places the workpiece in an alternating current coil, utilizing surface eddy currents for rapid heating, then cooling via water spray or self cooling. It offers fast heating, high efficiency, minimal deformation, easy automation, and precise control of hardened layer depth through frequency adjustment.
Flame hardening uses high temperature flame to heat the workpiece surface to quenching temperature, followed by immediate water cooling. It features simple equipment, low cost, and flexibility, but suffers from poor control over temperature and layer depth, prone to overheating and inconsistent product quality.
Laser or electron beam hardening scans the surface with high energy density beams, heating the layer rapidly and relying on the workpiece's thermal conductivity for self cooling. It delivers ultra high energy density, minimal deformation, and precise processing of complex local areas, producing a fine grained, high hardness layer suitable for strict deformation requirements.
II. Chemical Heat Treatment Technology
Chemical heat treatment heats workpieces in an active medium to infiltrate elements like carbon, nitrogen, and boron into the surface, modifying its chemical composition and microstructure for superior performance. Mainstream processes include carburizing, nitriding, carbonitriding, boronizing, and metal diffusion coating.
Carburizing treats low carbon steel in a carbon rich medium at high temperature, forming a high carbon surface layer. Subsequent quenching and low temperature tempering yield a deep hardened layer with high hardness and wear resistance, while maintaining core toughness.
Nitriding applies to alloy steel containing specific elements, heating in a nitriding medium at low temperature to form high hardness nitrides. It offers low temperature, minimal deformation, high surface hardness, and excellent wear, fatigue and corrosion resistance, but has a shallow layer and long processing cycle.
Carbonitriding infiltrates both carbon and nitrogen at a temperature between carburizing and nitriding. It combines their advantages: faster processing, small deformation, and improved wear resistance and fatigue strength.
Boronizing and metal diffusion coating form extremely hard compound layers on the surface, endowing materials with excellent abrasive wear resistance and anti galling performance.
III. Surface Coating and Deposition Technologies
These technologies strengthen workpieces by applying a high hardness, wear resistant coating or plating distinct from the substrate. Mainstream methods include Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), thermal spraying, electroplating, and electroless plating.
PVD vaporizes coating materials via physical means in a vacuum, depositing thin films on the surface. It operates at low temperatures, causing minimal deformation, with high coating hardness, regular appearance, and good substrate bonding.
CVD forms solid films through gas phase reactions at high temperatures. It produces dense, uniform coatings with strong substrate adhesion, suitable for complex shaped workpieces, but high temperatures may cause deformation and core softening.
Thermal spraying sprays molten or semi molten materials onto the surface via high speed airflow. It supports a wide range of materials and thick coatings, but has lower bonding strength than PVD and CVD and potential porosity.
Electroplating uses electrolysis, while electroless plating relies on chemical reactions to deposit metal or alloy coatings. Hard chrome plating offers excellent wear resistance, and electroless nickel plating has uniform thickness with heat treatable hardness.
IV. Surface Deformation Strengthening Technology
This method uses mechanical means to induce plastic deformation in the workpiece surface, forming a work hardened layer and residual compressive stress to enhance fatigue strength and stress corrosion resistance. Key processes include shot peening and rolling or extrusion strengthening.
Shot peening sprays high speed projectiles onto the part surface, inducing plastic deformation and residual compressive stress. It significantly improves fatigue strength with simple operation and low cost, enjoying wide application.
Rolling or extrusion strengthening applies pressure via hard rollers or balls to the part surface, inducing plastic deformation and compressive stress. It not only boosts fatigue strength but also reduces surface roughness, suitable for strengthening specific areas.
Understanding these core technologies enables targeted selection of the optimal surface hardening solution for specific application demands.
