Carburizing is a case hardening process in which a metal part or component of low carbon content is heated in a carbon-rich gas atmosphere. The process of heating the metal component in a high carbon environment allows for diffusion of carbon atoms directly into the surface of the part that needs to be case hardened. The amount of carbon and resultant case depth that gets diffused into the metal surface depends upon the carbon potential of the atmosphere, the temperature at which it is heated and the time it is exposed to that temperature. Higher temperatures, longer carburizing cycle times, and higher carbon potentials will increase the amount of carbon diffused into the surface and the depth of case. The hardening of both case and core material does not actually occur until the metal is rapidly quenched. After quenching, carburized parts are tempered down to meet the customer requirements for case and core hardness. Tempering dramatically improves ductility and toughness with a minor loss in hardness and strength.

Carburizing Properties

Increases surface hardness

Improves wear resistance and fatigue strength

The core of the material being hardened is tough and ductile

Case depths can be varied from .002″ to .250″ depending on application and intended use

The hardening of steels and irons involves heating a material to a temperature above its austenitizing temperature, which converts the material structure to austenite. After the material has been fully transformed to austenite, it is rapidly quenched to transform the material structure to martensite which increases the hardness and strength of the material to its highest possible level. After quenching, the material is tempered down to the desired final hardness and strength level by re-heating to a lower sub-critical temperature which serves to substantially increase ductility and toughness as the hardness and strength are gradually reduced.

Neutral hardening is performed in a neutral atmosphere where there is no net gain or loss of carbon, nitrogen or other elements to the material’s surface. This is typically used for finished or near-finished parts where decarburization of the surface is not allowed.

Open fire hardening is performed in air atmosphere furnaces where carbon can exit the material’s surface by reacting with air causing surface decarburization to occur. This is often performed on raw materials or rough parts where all surfaces will be ground or machined after hardening, which will remove the decarburized surface layer.

Properties and Areas of Application of Hardened Stainless Steel

Hardened stainless steel used in fields of application, where high demands are placed on corrosion resistance and wear protection. We produce a surface which is up to 5 times harder and moves the operating limits of the previously soft steel.

The areas of application are varied and comprise the consumer goods and household goods sector, mechanical engineering and construction, medical technology, automotive and components industry.

Several different methods and types of equipment can be selected for neutral hardening. The processing parameters, atmospheres used, and particular furnace type is chosen based on the particular steel or iron grade requiring hardening and if the quantity of parts to be treated is an individual piece, small lot, or a high volume production order.

Carbonitriding is a surface case hardening process that produces a thin, high hardness case and is a modified form of gas carburizing. This modification consists of introducing ammonia into the carburizing atmosphere to allow for nitrogen from the ammonia to be added to the carburized case as it forms. This thermochemical treatment diffuses both carbon and nitrogen into the surface of the component simultaneously. Carbonitriding is typically carried out at lower temperatures with shorter cycle times, resulting in shallower cases than most carburizing processes. Carbonitrided case depths will typically range from .002″ to .030″. The addition of nitrogen to a carburized case increases the hardenability of the carburized case. This allows plain carbon and lower alloy steels to be case hardened successfully with full transformation to martensite in order to maximize the case hardness and obtain the best possible mechanical properties. This process is typically selected for case hardening plain carbon and low alloy steels that have poor hardenability and would not normally fully transform to martensite during quenching.

Carbonitriding Properties

Increases surface hardness

Improves wear resistance and fatigue strength

The core of the material being hardened is tough and ductile

Case depths are normally .002″ to .030″ inch deep depending on application and intended use

Can be used on simple or complex parts

Can be performed in batch equipment for low to medium volume work or in continuous equipment for high volume work

Typically works best with plain carbon and low alloy steels (10xx, 11xx, 12xx, 13xx, 15xx, 40xx grades) that have a low carbon content of 0.05% to 0.30% C

Surface of parts should be clean and free of rust or scale

Carbonitriding Applications

Parts which require increased surface wear resistance and fatigue strength

Parts made from low carbon content and low hardenability steels

Reduced hardness, tensile strength, and yield strength

Easier machining and forming of materials

Reduced residual stresses

No effects of cold-work hardening

Recrystallization can eliminate directionality of rolled or forged microstructures and create fine equiaxed grains

Homogenization of grain structure to dissipate alloy segregation present in as-cast structures

Annealing Applications

Shaping

Stamping

Forming

Hydroforming

Bending

Machining

Castings

Forgings

The functions of normalizing may overlap with or easily be confused with those of annealing, hardening, and stress relieving; however, they are not interchangeable, and the final use of the product must be considered when determining which method to use. Normalization may increase or decrease the strength and hardness of metal in a given product form, depending on the thermal and mechanical history of the product.

Normalizing Process Overview

Normalizing heat treatment helps to remove impurities and improve ductility and toughness. During the normalizing process, material is heated to between 750-980 °C (1320-1796 °F). The exact heat applied for treatment will vary and is determined based on the amount of carbon content in the metal.

Depending on the mechanical properties required, normalizing may be substituted for conventional hardening when the size or shape of the part is such that liquid quenching might result in cracking, distortion, or excessive dimensional changes. Thus, pieces that are of complex shape or that incorporate sharp changes in the section may be normalized and tempered, provided that the properties obtained are acceptable.

The Outcome

Normalizing will typically produce a uniform pearlitic structure in combination with either ferrite grains or grain-boundary carbides present depending on the base material’s carbon content.

Improved machinability, grain-structure refinement, homogenization, and reduction of residual stresses are the primary reasons that normalizing is performed. Homogenization of castings by normalizing may be done to break up or refine the as-cast dendritic structure and facilitate a more uniform response to subsequent hardening. Similarly, for wrought metals, normalization can help reduce banded grain structure due to hot rolling, as well as large grain size or mixed large and small grain size due to forging practice.

Materials Suitable for Normalizing

Aluminum

Brass

Copper

Iron alloys

Nickel alloys

Steel

Normalizing has broad practical applications across industries, including:

Aerospace

Agriculture

Automotive

Energy

Heavy Equipment

Bending, welding, grinding, rolling, cold working, solidication, quenching, stamping, and other operations, can introduce new stresses into the structure of a metal part. Stress relieving is a heat treatment process designed to relieve residual stresses caused any such manufacturing process. As these stresses cause changes in the granular structure of the metal, they can cause challenges in working with a metal component in the future.

Stress relieving steel or the stress relief heat treatment of other metals and alloys becomes an important step in the manufacturing process to provide a final quality product. Stress-relief heat treating is the uniform heating of a structure to a suitable subcritical temperature below the austenitic transformation range. After the desired temperature is reached, it is held for a predetermined period, followed by uniform cooling.

This process must be controlled based on the metal or alloy. The temperature reached, and duration of the heating, and the cooling process precisely based on the material being treated.

Anodizing is an electrochemical process that converts the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish. Aluminum is ideally suited to anodizing, although other nonferrous metals, such as magnesium and titanium, also can be anodized. The anodic oxide structure originates from the aluminum substrate and is composed entirely of aluminum oxide.

This aluminum oxide is not applied to the surface like paint or plating, but is fully integrated with the underlying aluminum substrate, so it cannot chip or peel. It has a highly ordered, porous structure that allows for secondary processes such as coloring and sealing. Typical standard clear and color anodizing creates an aluminum oxide film in the range of .0002 to .0008 inches, (.005 to .020 mm), on each surface. Hard anodizing is typically in the range of .0005 to .003 inches, (.013 to .076 mm), the most common being .002 inches. (.051 mm).

The process of hard anodizing a part to .002 in. film thickness will therefore grow .001 in. on each surface or .002 in. in overall dimension.

Black oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys, zinc, powdered metals, and silver solder. It is used to add mild corrosion resistance, for appearance and to minimize light reflection. To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax.

One of its advantages over other coatings is its minimal buildup. Thickness of black oxide coating does not exceed 3*10-5 inch (0.75 µm), half of which is added to the part dimension and the second half penetrates into the part depth.

Powder coating is a high-quality finish found on thousands of products you come in contact with each day. Powder coating protects the roughest, toughest machinery as well as the household items you depend on daily. It provides a more durable finish than liquid paints can offer, while still providing an attractive finish. Powder coated products are more resistant to diminished coating quality as a result of impact, moisture, chemicals, ultraviolet light, and other extreme weather conditions.

In turn, this reduces the risk of scratches, chipping, abrasions, corrosion, fading, and other wear issues. It’s tough. It looks great. And it lasts a long, long time. In addition to being durable, powder coating is an attractive choice due to environmental advantages. Thickness is 2 to 5 mils, the final cured thickness of the powder should be between 0.002 and 0.005 of an inch.

These chemical conversion coatings provide corrosion prevention on unpainted items, and they improve adhesion of paint finish systems on aluminum and aluminum alloys. Metal surface coatings of this type may be used, for example, on tanks, tubing, and component structures where paint finishes are not required for interior surfaces but are required for the exterior surfaces. Thin coating in the range of 0.00001-0.00004 inches in thickness.

Nickel electroplating is a technique of electroplating a thin layer of nickel onto a metal object. The nickel layer can be decorative, provide corrosion resistance, wear resistance, or used to build up worn or undersized parts for salvage purposes. The maximum thickness of electroless nickel plating is limited to approximately 0.1 mm.

Also referred to as galvanization—employs an electrical current to facilitate the application of a thin coat of zinc to the surface of a metal component. The zinc oxidizes, creating a protective layer of zinc oxide that prevents the base metal from being exposed to the surrounding environment. Mechanical plating consists of a flash coating of copper followed by the zinc coating. Coating thickness requirements specified in ASTM B695 range from 0.2 to 4.3 mils (5 to 110 µm). While thicker coatings are possible, the common thickness on commercial fasteners is 2 mils (50 µm).

Tinning is the process of thinly coating sheets of wrought iron or steel with tin, and the resulting product is known as tinplate. The term is also widely used for the different process of coating a metal with solder before soldering. Standard thicknesses from 0.0001″ to 0.00006.

Gold plating is often used in electronics, to provide a corrosion-resistant electrically conductive layer on copper, typically in electrical connectors and printed circuit boards. With direct gold-on-copper plating, the copper atoms tend to diffuse through the gold layer, causing tarnishing of its surface and formation of an oxide and/or sulphide layer. A layer of a suitable barrier metal, usually nickel, is often deposited on the copper substrate before the gold plating. The layer of nickel provides mechanical backing for the gold layer, improving its wear resistance. It also reduces the impact of pores present in the gold layer. Adds 0.50 microns.

Passivation of stainless steel is a chemical treatment with a specific acid formulation that removes free-iron or other surface contamination from the stainless steel while simultaneously promoting the formation of a passive chromium/nickel oxide layer to act as a barrier to further corrosion. Typically, passivation layer thickness is about 0.0000001” or 1/ 100,000th the thickness of a human hair.

Black Zinc Plating refers to the color of the chromate that is applied during the post plating process. It includes a layer of zinc applied to a parts surface followed by black chromate applied over the zinc. Black Zinc is commonly specified for its dark appearance and protective properties. The typical thickness is minimal, ranging from .0002” to .0006”.