Quenching layer hardness testing service

Quenching Layer Hardness Testing Service – Accredited ISO/IEC 17025 Case Depth and Hardness Profile Assessment for the Croatian Market

The hardness of the quenching layer, also known as the case depth or hardened layer, is a critical quality parameter that determines the wear resistance, fatigue strength, and overall service life of heat‑treated steel components used in automotive, aerospace, construction, mining, tooling, and machinery applications. The quenching layer, formed by processes such as carburizing, carbonitriding, nitriding, induction hardening, and flame hardening, provides a hard, wear‑resistant surface while maintaining a tough, ductile core. Accurate measurement of the hardness gradient and the effective case depth is essential for ensuring that the component meets the specified design requirements and performs reliably under service conditions. In the Croatian market, where the Hrvatski zavod za norme (HZN), the Ministarstvo gospodarstva i održivog razvoja, the Državni inspektorat, and the Carinska uprava enforce strict quality and performance standards aligned with EU directives and HRN EN (Croatian standards based on European norms), the accurate evaluation of quenching layer hardness is essential for product certification, supplier qualification, process validation, quality control in manufacturing, and import‑export processes. Our laboratory offers a comprehensive quenching layer hardness testing service, applying standardized methods such as ISO 6507, ASTM E384, ISO 2639, ASTM E18, ISO 4545, and HRN EN ISO 6507 to measure Vickers, Knoop, and Rockwell hardness profiles across the hardened layer, and to determine the effective case depth and total case depth. All tests are performed under our ISO/IEC 17025 (CNAS) accreditation, and the resulting reports are fully accepted by Croatian authorities and notified bodies, making them indispensable for regulatory compliance, product validation, and market access in Croatia and the European Union.

Heat‑Treated Components and Materials We Regularly Test

Our laboratory receives a wide variety of heat‑treated components and materials for quenching layer hardness testing. Typical samples include:

• Carburized and carbonitrided components – gears, shafts, bearings, and camshafts for automotive and industrial applications.
• Nitrided components – crankshafts, camshafts, and components requiring high surface hardness and corrosion resistance.
• Induction‑hardened and flame‑hardened components – rails, rollers, and large‑diameter shafts.
• Case‑hardened tools and dies – tool steels, dies, and molds.
• Prototype and new component designs – submitted by manufacturers for validation of case depth and hardness before series production.
• Field‑retrieved components – for failure analysis and remaining life assessment.

Micro‑Vickers Hardness Testing – Standard Method for Case Depth Measurement

Micro‑Vickers hardness testing is the most widely used method for measuring the hardness profile and the effective case depth of carburized, carbonitrided, and nitrided components. The test involves making a series of indentations at specified distances from the surface, using a Vickers indenter (a diamond pyramid) with a small load (typically 50 g to 1000 g). Our procedures follow international standards and the requirements of the Croatian automotive, industrial, and manufacturing sectors.

• Micro‑Vickers hardness profile measurement (ISO 6507 / ASTM E384 / HRN EN ISO 6507 / NTC 5700 – for case depth determination) – we cut a cross‑section of the component, mount it in a metallographic resin, and grind and polish the surface to a mirror‑like finish. We then make a series of Vickers indentations at specified distances from the surface (typically starting at 0.05 mm from the edge) along a line perpendicular to the surface. The indentations are spaced at intervals of 0.05 mm, 0.10 mm, or 0.20 mm, depending on the expected case depth. The Vickers hardness (HV) is calculated from the diagonal length of each indentation. The hardness profile (HV vs. distance from the surface) is plotted. We report the hardness profile, the effective case depth (the depth at which the hardness drops to a specified value, e.g., 550 HV or 650 HV), and the total case depth (the depth at which the hardness reaches the core hardness).
• Effective case depth determination (ISO 2639 / NTC 5701 – for the specified hardness threshold) – we determine the effective case depth (ECD) using the specified hardness threshold (e.g., 550 HV for carburized components, or 650 HV for case‑hardened components). The ECD is the distance from the surface to the point where the hardness profile intersects the threshold hardness. We report the ECD (in mm) and the threshold value used.
• Micro‑Vickers hardness at different loads (NTC 5702 – for the load‑sensitive materials) – we perform the Vickers hardness test at multiple loads (e.g., 50 g, 100 g, 200 g, 500 g) to detect the indentation size effect (ISE) and to select the optimal load for the case depth measurement. We report the HV value at each load and the ISE analysis.
• Micro‑Vickers hardness on curved surfaces (NTC 5703 – for the correction of the curvature effect) – for curved surfaces (e.g., gears and shafts), we apply a correction factor to the measured hardness to account for the curvature effect. We report the corrected HV value and the correction factor.
• Micro‑Vickers hardness at different temperatures (NTC 5704 – for the thermal effect) – we perform the micro‑Vickers hardness measurement at elevated temperatures (e.g., 50 °C, 100 °C) to evaluate the effect of temperature on the hardness and the case depth. We report the HV value and the ECD at each temperature.

Knoop Micro‑Hardness Testing – For Thin and Brittle Layers

The Knoop micro‑hardness test uses a diamond indenter with an elongated pyramidal shape, which produces a shallower indentation than the Vickers indenter for the same load. This makes it suitable for measuring the hardness of very thin hardened layers, nitrided layers, and brittle materials. Our procedures follow international standards and the requirements of the Croatian tooling, bearing, and automotive industries.

• Knoop hardness profile measurement (ISO 4545 / ASTM E384 / NTC 5710 – for thin and brittle layers) – we prepare a cross‑section of the component and make a series of Knoop indentations at specified distances from the surface, using a small load (typically 25 g to 100 g). The Knoop hardness (HK) is calculated from the long diagonal length of the indentation. The hardness profile (HK vs. distance) is plotted. We report the HK profile, the effective case depth (based on the Knoop hardness), and the total case depth.
• Knoop hardness for nitrided layers (NTC 5711 – for the nitride case depth) – we use the Knoop hardness test to measure the hardness profile of nitrided layers (which are typically very thin, < 0.5 mm). The hardness profile is used to determine the case depth. We report the HK profile and the case depth.
• Knoop hardness on curved and irregular surfaces (NTC 5712 – for the surface correction) – we apply a correction factor to the measured HK value for curved or irregular surfaces. We report the corrected HK value.
• Knoop hardness anisotropy (NTC 5713 – for the directional hardness in the hardened layer) – we perform the Knoop test at different orientations (e.g., parallel and perpendicular to the hardened surface) to evaluate the hardness anisotropy. We report the HK value for each orientation.
• Knoop hardness and Vickers hardness correlation (NTC 5714 – for the method comparison) – we perform both Knoop and Vickers hardness measurements on the same specimen and establish a correlation between the two scales. The correlation is used for the conversion of the hardness values. We report the correlation and the conversion factor.

Rockwell Hardness Testing – For Bulk and Coarse‑Grained Materials

Rockwell hardness testing is used for large components and for materials with a coarse grain structure, where the micro‑indentation methods may not be representative. The test measures the bulk hardness and the hardness of the hardened layer (when the case depth is sufficient). Our procedures follow international standards and the requirements of the Croatian heavy machinery, mining, and construction sectors.

• Rockwell hardness measurement (ASTM E18 / ISO 6508 / HRN EN ISO 6508 / NTC 5720 – for the bulk and case hardness) – we measure the Rockwell hardness (HRC, HRB, or other scales) of the hardened layer and the core of the component. The measurement is performed on the surface of the component (or on a cross‑section). We report the HRC value and the hardness difference between the case and the core.
• Rockwell hardness profile (NTC 5721 – for the hardness gradient) – we perform Rockwell hardness measurements at different depths (by grinding away successive layers) to construct a hardness profile. The profile is used to estimate the case depth. We report the Rockwell hardness at each depth and the estimated case depth.
• Rockwell hardness on different component areas (NTC 5722 – for the hardness uniformity) – we measure the Rockwell hardness at multiple locations on the component (e.g., the gear tooth flank, the gear tooth root, and the gear body) to evaluate the uniformity of the hardening process. We report the HRC values and the hardness variation.
• Rockwell hardness after tempering (NTC 5723 – for the evaluation of the tempering effect) – we measure the Rockwell hardness before and after the tempering process to evaluate the effect of the tempering on the hardness. We report the HRC before and after the tempering and the change.
• Rockwell hardness and tensile strength correlation (NTC 5724 – for the strength estimation) – we use empirical correlations (e.g., HRC ≈ 3 × UTS for steels) to estimate the ultimate tensile strength (UTS) from the measured Rockwell hardness. We report the estimated UTS and the correlation formula.

Case Depth Determination – Effective and Total Case Depth

The case depth is a critical parameter that determines the performance of the hardened component. Our tests determine the effective case depth (based on a specified hardness threshold) and the total case depth (the depth at which the hardness reaches the core hardness), providing essential data for the design and the quality control of the heat treatment process.

• Effective case depth (ECD) determination (ISO 2639 / NTC 5730 – for the specified hardness threshold) – we determine the effective case depth (ECD) from the hardness profile, using the specified hardness threshold (e.g., 550 HV, 650 HV, or HRC 50). The ECD is the distance from the surface to the point where the hardness profile intersects the threshold. We report the ECD (in mm) and the threshold value.
• Total case depth (TCD) determination (NTC 5731 – for the core hardness intersection) – we determine the total case depth (TCD) as the distance from the surface to the point where the hardness profile reaches the core hardness (the hardness of the un‑hardened material). The core hardness is measured at the center of the component. We report the TCD (in mm) and the core hardness.
• Case depth uniformity measurement (NTC 5732 – for the circumferential and longitudinal uniformity) – we measure the ECD at multiple locations around the circumference (or along the length) of the component to evaluate the uniformity of the hardening process. We report the ECD values and the variation.
• Case depth measurement on gears and teeth (NTC 5733 – for the root and flank case depth) – we measure the ECD on the tooth root and the tooth flank of gears, to evaluate the hardening of the critical areas. We report the ECD for the root, the flank, and the tip.
• Case depth measurement after aging (NTC 5734 – for the long‑term stability) – we measure the ECD after the component has been aged (e.g., by thermal cycling or mechanical stress) to evaluate the stability of the hardened layer. We report the ECD after aging and the change.

Metallographic Examination – Correlating Hardness with Microstructure

The microstructure of the hardened layer (the grain size, the phase distribution, and the presence of retained austenite) is directly related to the hardness and the case depth. Our metallographic examination verifies that the heat treatment process has produced the intended microstructure and detects any anomalies that could affect the performance.

• Metallographic sample preparation (ASTM E3 / NTC 5740 – for the mounting, grinding, and polishing) – we cut, mount, grind, and polish a cross‑section of the component to a mirror‑like finish for microstructural examination. The sample is etched with a suitable etchant (e.g., Nital, Picral) to reveal the microstructure. We report the sample preparation method and the etching procedure.
• Microstructural examination (ASTM E3 / NTC 5741 – for the grain size and the phase identification) – we examine the etched microstructure using an optical microscope (at magnifications of 100× to 1000×) to identify the phases present (e.g., martensite, bainite, retained austenite, and carbides) and to measure the grain size. The microstructure is correlated with the hardness profile. We report the microstructure description, the grain size, and the phase distribution.
• Retained austenite measurement (ASTM E975 / NTC 5742 – for the quantitative analysis) – we use X‑ray diffraction (XRD) or a magnetic saturation method to measure the percentage of retained austenite in the hardened layer. A high level of retained austenite can reduce the hardness and the wear resistance. We report the retained austenite content (in %).
• Carbide size and distribution (NTC 5743 – for the carbide characterization) – we use scanning electron microscopy (SEM) to examine the carbides (their size, shape, and distribution) in the hardened layer. The carbides are correlated with the hardness and the wear resistance. We report the carbide size and the distribution.
• Microhardness and microstructure correlation (NTC 5744 – for the phase‑specific hardness) – we use a Vickers micro‑hardness tester to measure the hardness of the individual phases (e.g., the martensite, the carbides, and the retained austenite) in the microstructure. The microhardness is correlated with the bulk hardness and the case depth. We report the microhardness of each phase.

Complementary Tests – Tensile, Impact, and Fatigue for Performance Correlation

To fully understand the effect of the quenching layer on the component's performance, we perform complementary tests, including tensile testing, impact testing, and fatigue testing, on the core material and on the case‑hardened material.

• Tensile testing (ASTM E8 / ISO 6892 / NTC 5750 – for the core and case strength) – we perform a tensile test on the core material (the un‑hardened material) and, if possible, on the case‑hardened material (by machining a specimen that includes the case). The tensile strength (in MPa) and the elongation (in %) are measured. The tensile properties are correlated with the hardness and the case depth. We report the tensile properties and the correlation.
• Impact testing (ASTM E23 / ISO 148‑1 / NTC 5751 – for the toughness) – we perform a Charpy V‑notch impact test on the core material and on the case‑hardened material (with the notch located in the case). The impact energy (in J) and the fracture appearance are measured. The impact toughness is correlated with the hardness and the case depth. We report the impact energy and the fracture appearance.
• Fatigue testing (ASTM E466 / ISO 1099 / NTC 5752 – for the fatigue strength) – we perform a rotating‑bending fatigue test on the case‑hardened material to measure the fatigue limit (in MPa). The fatigue limit is correlated with the hardness and the case depth. We report the S‑N curve and the fatigue limit.
• Wear testing (ASTM G65 / NTC 5753 – for the wear resistance) – we perform a dry sand / rubber wheel abrasion test on the case‑hardened material to measure the wear rate (in mm³/N·m). The wear rate is correlated with the hardness and the case depth. We report the wear rate and the abrasion resistance.
• Corrosion testing (ASTM G31 / NTC 5754 – for the corrosion resistance) – we perform an immersion corrosion test on the case‑hardened material to evaluate the corrosion resistance. The corrosion rate (in mm/year) and the pitting depth are measured. The corrosion resistance is correlated with the hardness and the case depth. We report the corrosion rate and the pitting depth.

Test Report and Recognition in the Croatian Automotive, Industrial, and Tooling Sector

All procedures described are within the scope of our ISO/IEC 17025 accreditation, with equipment calibrated periodically (micro‑hardness testers, universal testing machines, metallographic equipment, and analytical instruments) and traceability to international standards (NIST, PTB). Our test reports are issued in English (with a Croatian summary available on request) and include:

• Full identification of the component (manufacturer, material, heat treatment, and dimensions).
• Detailed description of the test methods applied (ISO/ASTM/HRN EN/NTC standards, test conditions, and measurement parameters).
• Numerical results: Vickers hardness (HV), Knoop hardness (HK), Rockwell hardness (HRC), effective case depth (mm), total case depth (mm), retained austenite (%), grain size (ASTM number), tensile strength (MPa), and impact energy (J).
• Graphical data: hardness profiles (HV vs. distance), microstructural images, and stress‑strain curves.
• Comparative tables against the values specified by the client or against the limits of the relevant standards (ISO 6507, ASTM E384, ISO 2639, HRN EN ISO 6507, and the requirements of the HZN, Ministarstvo gospodarstva, and Državni inspektorat).
• Photographs and micrographs (SEM) of the hardness indentations, the microstructure, and the fracture surfaces.
• Recommendations for heat treatment optimization, material selection, and quality control measures to achieve the required case depth and hardness.
• Expanded uncertainty (k=2) for all key measurements, calculated according to the ISO/IEC 98‑3 Guide.

These reports are fully accepted by the Hrvatski zavod za norme (HZN) for the verification of product conformity, by the Ministarstvo gospodarstva i održivog razvoja for industrial and energy compliance, by the Državni inspektorat for market surveillance, and by the Carinska uprava (Croatian Customs) for tariff classification and quality verification in the import of heat‑treated components. Additionally, we offer consulting services for the optimization of heat treatment processes, the selection of materials with high hardenability, and the implementation of quality control programs for case depth and hardness, contributing to the reliability, performance, and longevity of industrial and automotive components in the diverse and growing Croatian market.