Surface Hardening

Surface Hardening: The Ultimate Guide

Metal parts are not inherently strong. Therefore, it is critical to improve their performance through surface hardening. This guide takes you through all vital aspects of surface hardening – let’s dive right in.

What Is Surface Hardening?

It is the process of reinforcing metal surface by enhancing the outer part while the inner core retains all the original properties. The essence is to improve the metal surface’s hardness, fatigue life, and general wear resistance.

Materials Suitable For Surface Hardening

Surface Hardening Of Steel – This process enables steel to extend its service life in different applications by making it relatively resistant to abrasion and mechanical impacts.

Surface Hardening Aluminum – Often aluminum is commonly used in various applications. Surface hardening thus increases its tensile strength, which improves its hardness value.

Surface Hardening Cast Iron – Usually, a minimum of 0.4-0.5% combined carbon content is used in hardening cast iron with short heating cycles.

Surface Hardening Copper – It is a malleable and ductile material thus often requires some surface hardening to suit particular applications.

Materials Suitable For Surface Hardening
Advantages Of Surface Hardening

Advantages Of Surface Hardening

Improves metal’s strength and durability: This process develops mechanical strength while retaining a soft core of the metal, thus improving wear resistance and fatigue life of components.

Enhances toughness and mechanical resistance: The metal becomes resistant to different mechanical impacts since it develops secondary toughness.

Enables easy machinability of metal: It becomes easier to machine the metal since it is hardened using specific additives suitable for particular machining operations.

Allows easy customization: The process makes the metal easy to machine; hence you can customize the part into the desired shape.

Cost-saving: It reduces heat treatment costs.

Surface Hardening History

Early iron melting produced one metal layer with low carbon (wrought iron) and another layer with a high carbon (cast iron). Thus, case hardening entailed packing iron in a compound with high carbon and heated to facilitate carbon transfer to iron surface.

It formed a thin higher carbon steel layer on surface with carbon content decreasing gradually further from surface. The resulting product was tough with hardness and wear resistance of outer material and softer iron core. However, this process evolved over time, and today you can make homogeneous steels of low to high carbon content.

Surface Hardening History

Surface Hardening Processes

Surface Hardening Processes
Surface Hardening Processes

It is available in a wide range of types, but the common ones include the following;

Carburizing

In this method, the metal material is heated for a set period in the presence of carbon environment then quenched. This allows carbon to be deposited on surface of metal material. The carbon penetrates the metal surface as it strengthens it.

Primary, the surface material of the metal is hardened by adding carbon compounds. It is imperative to conduct this process in a controlled environment to prevent oxygen from penetrating and altering the metal hardening.

Nitriding

This metal surface hardening process involves heating the material and exposing it to atomic nitrogen, which penetrates the metal. The exposure of the metal to atomic nitrogen forms a thin and hard case leading to formation of nitrides.

Essentially, the process requires strong nitride-forming compounds to be present in the metal. In this case, atomic nitrogen reacts with the metal to enhance its resistance and hardness. Nitriding offers a higher hardness degree but is only suitable for specific types of metals. The main advantage of this process is that it produces parts that can be quenched, tempered, and machined before nitriding.

Boriding

In some instances, it is known as boronizing. It is typically a thermochemical diffusion process relatively similar to nitrocarburizing.  In this process, the boron atoms diffuse into a substrate to yield tough and wear-resistant surface layers.

Ordinarily, this process needs a relatively high temperature for treatment of about 1073-1323 K and should last between 1-12 hours. It is a suitable surface hardening process that can be applied to a wide range of metal materials and still attain desired results.

Carbonitriding

It is a surface hardening treatment which introduces carbon and nitrogen to metallic components. This process is more or less identical to carburizing in that the austenitic composition is altered, and high surface hardness is yielded by quenching.

Carbonitriding is usually applied to inexpensive and easily machined metals to impart the surface properties of costlier and complex metals without quenching. It often results in less distortion and minimizes risks of cracking the work. Carbonitrided parts usually have a surface hardness range from 55-62 HRC. Mostly, it is performed on parts used in different applications where surface and structural fatigue is a concern.

Induction Hardening Process

Induction Hardening Process
Induction Hardening Process

This type of surface hardening uses induction coils to provide a swift means of heating metal, then cooled rapidly using water. Ideally, carbon content of 0.3-0.6 wt% is required for this hardening type to create martensite on the surface.

Martensite is typically a hard metastable structure with a crystal structure of body-centered tetragonal. Induction hardening yields a tough, immensely wear-resistant surface with good bending fatigue strength and contact load.

Technically, heating is attained by placing metal in magnetic field generated by a high-frequency changing current passing through an inductor. The currents dissipate energy to produce heat by flowing against an imperfect conductor resistance.

Induction enables metal heating rapidly before heat can penetrate any distance into the material. It makes the surface immensely resistant to wear since the core component remains unaffected by its physical properties and treatment. Metal surface is then quenched to harden it and usually used without additional tempering.

Laser Hardening

Laser Hardening
Laser Hardening

It is a surface hardening method that utilizes a laser beam to give a rapid means of heating metal then cooled rapidly. This process creates a case of martensite layer on the surface while the inner core of the metal maintains the original elasticity and hardness.

Heat generated by laser light absorption is regulated to prevent melting thus used in the selective local surface austenitization. After removing the specific heat source from interaction, zone is when the self-quenching phenomenon applies.

Surface layer absorbs thermal energy, which is quickly distributed to the whole workpiece.

Laser hardening yields hard-wearing, wear-resistant surface. Additionally, thin surface zones heated and cooled faster, resulting in fine martensitic micro-structures even in metals with low hardenability. The main advantage of this surface hardening process is that it exhibits minimal processed parts’ deformations, and it is relatively fast.

Flame Hardening

Flame Hardening
Flame Hardening

This technique uses a single torch with a specially designed head to offer rapid metal heating means.  It entails rapid heating of the metal material with a direct high-temperature gas flame.

The surface layer of the heated metal material is heated above the transformation range and cooled at a rate that forms the desired hardening. Usually, most metals hardened through flame hardening are often in a range of 0.3-0.6% carbon.

They also have hardenability ideal for case depth required and quenchant used. The quenchant is typically sprayed on the material surface a short distance behind the heating flame. Immediate tampering is needed and can be done in a traditional furnace or by a flame-tempering method. One advantage of this process is that it makes localized hardening a possibility.

Electron Beam Hardening

Electron Beam Hardening
Electron Beam Hardening

This is a surface hardening technique that utilizes organized electrons column as the source of energy. Essentially, the bombardment of electron column on surface material generates heat, which increases until the material or surface attains the desired temperature.

Hence, it enables the material to attain excellent surface wear resistance while maintaining toughness and ductility in its core. Typically, this method is used for hardening objects with a thin surface layer by raising them to a specific temperature and allowing them to cool.

The unique aspect of this surface hardening process is its ability to harden the outermost material layer only. Hence, it means core material of the metal retains its original chemical and mechanical properties.

Moreover, this process can be used for hardening material thickness of less than 0.1mm.

Technically, it entails exciting and emitting electrodes from a tungsten-made electrode. As such, thousands of volts are moved through the filament, which emits the electrons.

Magnetic fields are used to organize and focus the emitted electrons during the electron beam hardening process. The beam is also directed across the metal during this process.

Disadvantages Of Surface Hardening

Relatively Less Hardness Depth
Relatively Less Hardness Depth

Mostly, the hardness is applied as a thin layer on the surface material of the metal. Thus, it can compromise the quality of some applications, especially those that require deeper hardness.

Suitable For Selected Metals
Suitable For Selected Metals

The suitability of this process is limited to specific metals.

Comparatively Costly
Comparatively Costly

Depending on the actual surface hardening process you use, it can be expensive, especially for adoption, due to formation of a thin layer.

Requires A Lot Of Care
Requires A Lot Of Care

Ordinarily, there will hardly be a post-heat treatment process for parts that go through surface hardening. Thus, it implies thorough care must be taken, especially regarding the inner core of the metal.

Surface Hardening Applications

Automotive Industry
Automotive Industry

Common parts in the automotive industry hardened using this process include bearings, gears, engines, gearboxes, driveshaft, etc.

Aerospace Industry
Aerospace Industry

In this industry, accreditation and certification are fundamental. Parts used in this sector that are surface hardened include engine blades, valves, bearings, cylinders, landing gear, etc.

Fitting Industry
Fitting Industry

The notable surfaced hardened parts in this sector include screws, bolts, washers, nuts, rivets, etc.

Machinery
Machinery

Engine camshafts, fasteners, bolts, and firing pins are some surface hardened parts in this sector.

Household Items
Household Items

Cast iron pans, kitchen utensils, fireplace items, and related equipment

People Also Ask:

Is Surface Hardening Similar To Case Hardening?

The main difference between these processes is that case hardening forms a thin layer on a metal surface. On the other hand, surface hardening merely hardens the outer layer and maintains the core soft throughout the whole process.

Which Surface Hardening Technique Is Popular?

Carburizing

Is Tempering Same As Surface Hardening?

No.

Tempering is a relatively low-temperature heat treatment process often performed after surface hardening process to attain desired hardness to toughness ratio.

More Resources:

Electrolytic Polishing

Powder Coating On Metal

Metal Anodizing

Metal Plating

Metal Painting

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