Carbonitriding Layer Testing

Carbonitrided layer inspection mainly conducts scientific testing and analysis on the effective case depth determination, surface hardness testing, case structure analysis, carbon and nitrogen concentration gradient analysis, core hardness and structure examination and surface brittleness evaluation, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Effective case depth determination: This is the core item of the carbonitrided layer inspection, referring to the vertical distance from the part surface to the specified hardness value (usually 550 HV). It directly determines the wear resistance, fatigue strength and load-bearing capacity of the parts, and is the key indicator of the process control and quality acceptance, usually measured according to the standards such as GB/T 9450 or ISO 2639.

Surface hardness testing: Used to evaluate the ability of the part surfaces to resist the plastic deformation and wear. Usually, the Rockwell hardness testers (HRC) or Vickers hardness testers (HV) are used for the multiple-point measurement on the surfaces, requiring the hardness values to be uniform and reach the technical requirements. The excessively high surface hardness may cause the brittleness, and the comprehensive judgement needs to be made in combination with the core hardness.

Case structure analysis: The micro-structures of the case cross-sections are observed through the metallographic microscopes. The morphologies, amounts and distributions of the carbonitrides, the retained austenite contents, as well as whether there are the defective structures such as the black structures and network compounds need to be evaluated. The qualified structures are the basis of guaranteeing the properties.

Carbon and nitrogen concentration gradient analysis: The equipment such as the electron probe microanalysers (EPMA) or glow discharge optical emission spectrometers (GDOES) is used to analyse the content changes of the carbon and nitrogen elements point by point from the surfaces to the cores. This gradient curve reflects the diffusion conditions of the infiltrated elements and is the direct basis of optimizing the process parameters (such as the temperatures, times and atmospheres).

Core hardness and structure examination: The hardness and micro-structures of the part cores (usually referring to the areas below the cases not affected by the thermochemical treatment) are tested. The core properties (such as the strength and toughness) play an important supporting role for the overall mechanical properties of the parts, especially the ability to resist the impact loads.

Surface brittleness evaluation: Mainly evaluates the brittleness tendencies caused by the carbonitrides in the cases (especially when coarse or network-distributed). The common methods include the Vickers hardness indentation crack rating method (according to GB/T 9451) or observing whether the surface spalling occurs through the impact and bending testing.

Testing Scope

Gear parts: The high-load transmission parts such as the automobile transmission gears and industrial reducer gears are the main application objects of the carbonitriding. The testing emphases are ensuring that the effective case depths are sufficient to resist the contact fatigue, the surface hardnesses are high and uniform, and there are no harmful structures to prevent the pitting and tooth fracture.

Shaft and cam parts: Including the engine camshafts, machine tool spindles and various pins, etc. These parts bear the alternating bending or contact stresses, and the testing needs to pay attention to the matching of the case depths with the part cross-section sizes, as well as the surface wear resistance and fatigue resistance.

Fasteners and tools: Such as the high-strength bolts, screws and some small tools and dies. The testing focuses on the surface hardnesses and case uniformity, to guarantee their wear resistance, anti-galling and sufficient tensile strengths, while controlling the deformation amounts.

Wear-resistant plates and guide rails: Applied to the occasions requiring the high surface hardnesses and good anti-scuffing abilities such as the dies and machine tool guide rails. In addition to the routine items, the testing scope also needs to particularly pay attention to the case uniformity under the surface flatness and the tribological properties.

Process development and quality arbitration: In the research and development stage of the new processes, the various parameters are systematically tested to establish the process-structure-property relationships; when the early failures or quality disputes occur, the comprehensive testing data are the key evidence of analysing the root causes and dividing the responsibilities.

In-service part failure analysis: The carbonitrided parts with the wear, spalling or fracture during the use are tested, and through the analysis of the case states at the failure positions (such as whether the depths are insufficient and whether the structures are deteriorated), the process or design problems are traced.

Testing Methods

Hardness method (arbitration method): According to GB/T 9450/ISO 2639, on the cross-sections perpendicular to the specimen surfaces, the Vickers hardness (test force usually 9.8N) is measured from the surfaces to the cores at the certain intervals, and the hardness-distance curves are drawn. The effective case depth is defined as the vertical distance from the surfaces to where the hardness drops to 550 HV. This method is accurate and intuitive, and is the common standard of judging the case depths.

Metallographic method: After the cutting, mounting, grinding and polishing, the specimens are etched with the specific etchants (such as the 3-4% nital solutions) and observed under the metallographic microscopes. The total case depths are estimated by measuring the distance from the surfaces to the obvious boundaries with the core structures. This method is fast and simple, but is greatly affected by the subjectivity, and is often used for the process control.

Chemical layer stripping analysis method: Through the electrolytic polishing or layer-by-layer turning, the cases are stripped from the surfaces layer by layer, and the chips of each layer are collected for the chemical analysis to determine the carbon and nitrogen contents, thereby obtaining the carbon and nitrogen concentration distribution curves. This method can precisely obtain the element gradients, but the operation is cumbersome and destroys the specimens.

Spectroscopic analysis method: The glow discharge optical emission spectrometers (GDOES) or electron probe micro-area analysers (EPMA) are used for the direct determination. The GDOES can quickly and continuously sputter the surfaces and analyse the changes of the element concentrations with the depths in real time, with the high precision and efficiency. It has become the important means of the modern testing.

Micro hardness gradient method: Similar to the hardness method of measuring the effective depths, but the measurement points are denser, used to precisely depict the hardness distribution profiles of the whole cases. This has the important value for studying the property gradients of the cases and optimizing the processes (such as the diffusion stage durations).

Non-destructive testing methods: Including the ultrasonic method, eddy current method and magnetic Barkhausen noise method, etc. These methods indirectly evaluate the case depths or hardnesses by measuring the acoustic and electromagnetic property changes caused by the cases, and are suitable for the fast screening of the batch parts on the production lines, but need to be calibrated with the destructive methods.

Testing Standards

Domestic Standards:

1. GB/T 9450, Steels — Determination and verification of the effective depth of carburized and hardened cases

2. GB/T 9451, Determination of total or effective thickness of thin surface hardened layers of steel parts

3. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

4. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

International Standards:

1. ISO 2639, Steels — Determination and verification of the effective depth of carburized and hardened cases

2. ISO 6507-1, Metallic materials — Vickers hardness test — Part 1: Test method

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Vickers/Knoop micro hardness tester: The core equipment, used to measure the effective case depths and surface hardnesses. The high-end models equipped with the automatic turrets and measurement systems can realize the automatic indentation, measurement and curve drawing from the surfaces to the cores, greatly improving the detection efficiency and accuracy, meeting the requirements of the ISO 6507 standard.

Metallographic microscope and image analysis system: Used to observe and analyse the micro-structures of the cases and cores. The modern digital microscopes combined with the image analysis software can not only take the high-detection-efficiency structure photos, but also carry out the quantitative measurement of the grain sizes and phase contents (such as the retained austenite), as well as the auxiliary measurement of the case depths.

Glow discharge optical emission spectrometer: The advanced equipment for the element depth profile analysis. It uses the radio frequency to excite the argon plasma to sputter the sample surfaces, and carries out the spectroscopic analysis of the sputtered atoms, which can obtain the precise concentration distribution curves of the carbon, nitrogen and other alloying elements from the surfaces to the substrates within a few minutes.

Electron probe X-ray micro-area analyser: The focused high-energy electron beams are used to bombard the sample micro-areas, exciting the characteristic X-rays for the qualitative and quantitative element analysis and the surface distribution and line scanning analysis. It is particularly suitable for the high-precision analysis of the element compositions of the tiny areas or specific phases (such as the carbonitrides) in the cases.

Automatic grinding and polishing machine and mounting machine: The pre-treatment equipment of preparing the high-quality testing specimens for the hardness method and metallographic method. The automatic grinding and polishing machines can ensure that the specimen cross-sections are flat, scratch-free and free of the heat effects; the mounting machines seal the irregular small specimens with the resins, convenient for the subsequent gripping and grinding and polishing operations.

Rockwell hardness tester and Leeb hardness tester: The Rockwell hardness testers (such as the HRC scales) are often used for the fast testing of the macro hardnesses of the part surfaces and cores. The portable equipment such as the Leeb hardness testers are suitable for the on-site non-destructive hardness sampling inspections of the large workpieces or assembled parts, but their results need to be converted to the routine hardness values.