Knowledge

Introduction

Surface treatment is a process method that artificially forms a surface layer with mechanical, physical and chemical properties different from those of the base material on the surface of the base material. The purpose of surface treatment is to meet the requirements of corrosion resistance, wear resistance, decoration or other special functions of the product. For metal castings, the surface treatment methods we commonly use are mechanical grinding, chemical treatment, surface heat treatment, and surface spraying, etc.

I.Surface Deformation Strengthening

Surface deformation strengtheningrefers to a surface strengthening process that causes plastic deformation on the surface of steel parts at room temperature to form surface compressive stress, thereby increasing the surface hardness and generating a favorable residual compressive stress distribution. This process is simple and cost-effective, and it is an important measure to enhance the fatigue resistance of steel parts and extend their service life.

Shot Peening

Shot peening is a technique that involves spraying a large number of high-speed moving shot pellets onto the surface of a part, similar to countless small hammers striking the metal surface, causing a certain amount of plastic deformation in the surface and subsurface layers of the part to achieve strengthening.

Application: It is used for parts with complex shapes after grinding, electroplating, and other processes.

Rolling Treatment

Rolling treatment involves using freely rotating hardened steel rollers to roll over the machined surface of steel parts, causing plastic deformation, flattening the rough peaks on the surface of the steel parts, and forming a favorable residual compressive stress, thereby improving the wear resistance and fatigue resistance of the workpiece.

Surface Swelling (Squeezing or Extrusion)

Surface swelling is a process where a steel ball or other shaped swelling tool with a diameter slightly larger than the hole diameter is squeezed through the machined inner hole of a workpiece at room temperature to obtain an accurate, smooth, and strengthened surface.

II.Surface Heat Treatment

Surface Quenching

Surface quenching is a heat treatment method that strengthens the surface of a part by rapidly heating the surface to austenitize it and then quenching it without changing the chemical composition or the core structure of the steel.

Induction Heating

A method that uses alternating current to induce a large eddy current on the surface of the workpiece, rapidly heating the surface.

Flame Heating

A method that directly heats the surface of the workpiece with an acetylene flame. It is low-cost but difficult to control the quality.

Laser Heating

A method that uses high-energy-density laser to heat the surface of the workpiece. It is highly efficient and of good quality.

Laser surface strengthening can be divided into laser phase transformation strengthening treatment, laser surface alloying treatment, and laser cladding treatment, etc.​​​​​​​

Bluing

A process where steel or steel parts are heated to an appropriate temperature in air-water vapor or chemical solutions to form a blue or black oxide film on the surface. Also known as blackening.

Phosphating

The process where workpieces (steel, aluminum, or zinc parts) are immersed in phosphating solution (a solution mainly composed of certain acid phosphates) to deposit and form a water-insoluble crystalline phosphate conversion film on the surface is called phosphating.

III. Chemical Surface Heat Treatment

Chemical surface heat treatment involves heating and holding a workpiece in a specific medium, allowing active atoms from the medium to penetrate the surface layer of the workpiece, thereby altering the chemical composition and structure of the surface layer and subsequently changing its properties.

Chemical heat treatment is also one of the methods to achieve a hard surface and tough core. Compared with surface quenching, chemical heat treatment not only changes the surface structure of steel but also its chemical composition. Depending on the elements being diffused in, chemical heat treatment can be classified into carburizing, nitriding, multi-element co-diffusion, and diffusion of other elements. The chemical heat treatment process consists of three fundamental steps: decomposition, absorption, and diffusion.

IV. Surface Coating Strengthening

Surface coating strengthening is a surface strengthening process that involves applying one or more layers of other metals or non-metals on the surface of metals through physical or chemical methods.

Objective: To enhance the wear resistance, corrosion resistance, heat resistance of steel parts or for surface decoration.

V. Surface Cleaning and Decoration

Polishing

Polishing is a finishing process that modifies the surface of a part. Generally, it can only achieve a smooth surface and cannot improve or even maintain the original machining accuracy. Depending on the pre-processing conditions, the Ra value after polishing can reach 1.6 to 0.008 mm.

Mechanical Polishing

It includes wheel polishing, barrel polishing, and vibratory polishing.

Electrochemical Polishing

Electrochemical polishing is similar to chemical polishing, but it also requires the application of direct current. The workpiece is connected to the positive terminal, causing anodic dissolution. It also utilizes the phenomenon that the protruding parts of the metal surface dissolve faster than the concave parts for polishing.

Coating

Coating Processes

Common coating processes include: brush coating, automatic dip coating, manual spraying (including high-pressure airless spraying), flow coating, curtain flow coating, fluidized bed coating, roller coating, and electrostatic spraying, etc.​​​​​​​

Vigorhas more than 18 years experience on surface treatment for castings, forgings and other metal parts.If you have any question and demand of products development or improve your supply chain, please feel free to contact us atinfo@castings-forging.com