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What are the methods for hardening the surface of materials?

Sometimes, people hope that the surface of materials can be harder, such as in situations where wear resistance is required, where fatigue strength needs to be improved, 

where corrosion resistance is required, where high-temperature oxidation resistance is required, and so on. What are the methods to improve the surface hardness of 

materials? Today, an article will introduce you to the whole picture in your mind, so that you can have a targeted approach when choosing the appropriate method next time.

1. Improve hardness through surface quenching.

Common methods include induction hardening, flame hardening, laser/electron beam hardening, etc. These methods do not change the chemical composition of the 

workpiece surface, but only achieve hardening by rapidly heating and cooling the surface structure to undergo phase transformation (such as martensitic transformation).

 However, the material itself is required to have a certain carbon content (such as medium carbon steel), possess hardenability and a certain degree of hardenability.

Let's take a closer look below.

Induction hardening: It is to place the workpiece in a coil with alternating current, generate induced eddy currents on the surface of the workpiece, and quickly heat it 

up, followed by rapid cooling (water spraying or self cooling). It has fast heating speed, high efficiency, small deformation, easy automation control, and relatively 

controllable hardening layer depth (high-frequency shallow layer, intermediate frequency middle layer, power frequency deep layer).

Flame quenching: Use high-temperature flames such as oxygen acetylene to spray the surface of the workpiece, quickly heating it to the quenching temperature, 

and then immediately spraying water for cooling. This method has simple equipment, low cost, and high flexibility, but the heating temperature and layer depth are 

difficult to control, making it prone to overheating and resulting in poor product quality stability.

Laser/electron beam quenching: using high energy density (laser beam or electron beam) to scan the surface of the workpiece, heating the surface layer at an 

extremely fast speed, and rapidly cooling by relying on the workpiece's own thermal conductivity (self-excited cooling). This method has extremely high energy 

density, minimal deformation, and can handle complex shaped local areas. The hardened layer has a finer structure and higher hardness, making it suitable 

for situations with strict deformation requirements.

2. Use chemical heat treatment to improve hardness

By heating the workpiece in a specific active medium, one or more elements (such as carbon C, nitrogen N, boron B, etc.) infiltrate the surface of the workpiece, 

changing its chemical composition and structure, thereby obtaining special properties. Common types of active media include carburizing, nitriding, 

carbonitriding, boronizing, and metalizing (such as chromizing and vanadizing). Let's take a brief look below.

Carbonization:

Heat low carbon steel (0.1-0.25% C) workpieces in carbon rich media (gas, solid, liquid) at 900-950 ℃ to allow carbon atoms to penetrate the surface layer,

 resulting in a high carbon surface layer (0.8-1.0% C), which is then quenched and low-temperature tempered. A deep (0.5-2mm) hardened layer with high 

hardness and good wear resistance can be obtained, while maintaining high toughness in the core.

Nitriding:

Heating steel parts containing alloy elements such as chromium, molybdenum, and aluminum in a nitriding medium such as ammonia (500-580 ℃) allows 

nitrogen atoms to penetrate the surface layer and form high hardness nitrides. Low processing temperature, minimal deformation, extremely high hardness 

(up to 1000-1200HV), good wear resistance, fatigue resistance, and corrosion resistance. But the depth is shallow (0.1-0.6mm) and the time is long.

Carbon nitrogen co infiltration:

Simultaneously infiltrate carbon and nitrogen atoms into the surface of the steel component at a temperature between carburizing and nitriding (800-860 ℃).

It combines the advantages of carburizing and nitriding, with faster speed than carburizing, smaller deformation, better wear resistance and fatigue strength.

Boronization and metalization (chromizing, vanadizing, etc.):

Penetrating boron or metal atoms into the surface of steel parts to form extremely hard borides or carbide layers (such as FeB, Fe2B, Cr7C3, VC). The 

surface hardness is extremely high (boron infiltration can reach 1200-2000HV, vanadium infiltration can reach 3000HV or more), and it has excellent wear

 resistance and anti bite performance.

3. Improve hardness through surface coating and deposition techniques

This type of method involves preparing a coating or plating layer on the surface of the workpiece that is completely different from its substrate material and 

has high hardness and wear resistance. There are mainly methods such as physical vapor deposition (PVD), chemical vapor deposition (CVD), thermal 

spraying, electroplating, and chemical plating.

Physical Vapor Deposition (PVD):

Under vacuum conditions, the coating material is vaporized by physical methods such as evaporation, sputtering, ion plating, and deposited on the 

surface of the workpiece to form a thin film. Such as TiN (golden yellow), TiCN (blue gray), CrN, etc. It has the characteristics of low processing temperature 

(200-500 ℃), small workpiece deformation, high coating hardness, beautiful appearance, and good bonding strength.

Chemical Vapor Deposition (CVD):

At higher temperatures (800-1000 ℃), solid thin films are generated and deposited on the surface of the workpiece through gas-phase chemical reactions.

 Such as depositing TiC, TiN, diamond films, etc. Characteristics: The coating is dense and uniform, with strong adhesion to the substrate, and can handle 

complex shaped workpieces. But high temperature may cause deformation of the workpiece and softening of the core tissue. Thermal spraying principle: 

Heating coating materials such as metals, ceramics, and metal ceramics to a molten or semi molten state, and spraying them onto the surface of the 

workpiece through high-speed airflow to form a coating. Characteristics: The coating material range is wide, the coating thickness is large, but the 

bonding strength is usually lower than that of PVD/CVD, and there may be pores.

Electroplating and Chemical Plating:

Electroplating is the use of electrolysis; Chemical plating is the process of depositing a layer of metal or alloy coating on the surface of a workpiece

 through chemical reactions, such as hard chromium plating or electroless nickel phosphorus (Ni-P) alloy plating. Features: Hard chrome plating with 

high hardness (800-1000HV) and good wear resistance; The chemical nickel plating layer is uniform, and the hardness can be improved by heat treatment.

4. Strengthening through surface deformation

This method is also very useful. It uses mechanical means to induce plastic deformation on the surface of the workpiece, forming a work hardening 

layer and residual compressive stress, thereby improving fatigue strength and stress corrosion resistance. Mainly including shot peening, 

olling/extrusion strengthening, etc.

Shot peening strengthening:

Spray a large number of high-speed projectiles (such as steel balls, glass balls, etc.) onto the surface of the parts to induce plastic deformation and

residual compressive stress. Significantly improve the fatigue strength of parts (up to 30% -50%), with simple operation and low cost.

Rolling/extrusion strengthening:

Apply pressure to the surface of the part (such as the shoulder and inner hole) using a hard roller or sphere to induce plastic deformation and 

compressive stress. Not only can it improve fatigue strength, but it can also reduce surface roughness. Specially suitable for the treatment of rounded

 corners and inner hole surfaces of shaft components.

Below is another table for your reference when choosing.