Nitriding Layer Depth Testing for Die-Casting Sleeves

Nitriding layer depth testing is a critical quality-control procedure for die-casting sleeves, shot sleeves, plunger bushings, and related hot-work tooling that depend on nitrided surfaces for wear resistance, thermal-fatigue resistance, and extended service life. This article explains the metallurgical structure of nitrided cases, the metallographic and microhardness methods used to measure case depth in accordance with ISO 18203, GB/T 11354, and ISO 6507-1, and provides practical guidance on sampling, test conditions, reporting, and typical acceptance criteria. It serves as a technical reference for design engineers, quality managers, heat-treatment specialists, and laboratory personnel responsible for verifying nitrided die-casting sleeve components.

Technical Overview

Die-casting sleeves — including shot sleeves (pressure chambers) for cold-chamber machines, plunger bushings, gooseneck liners for hot-chamber machines, and ejector sleeves — operate in one of the most demanding environments in metal-forming technology. They are exposed to molten aluminum at approximately 620 to 700 °C, high plunger sliding speeds, abrasive oxide particles carried by the melt, and severe thermal cycling between every shot. To withstand these conditions, sleeves are typically manufactured from hot-work tool steel (H13 / 1.2344), alloy steels such as 42CrMo4, or nitridable grades of ductile iron, and are subsequently surface hardened by gas nitriding, plasma (ion) nitriding, or salt-bath nitrocarburizing.

Nitriding is performed at 480 to 580 °C, below the transformation temperature of the steel, which produces a hard case without the quenching distortion associated with carburizing and hardening. The resulting case consists of two zones. The first is the compound layer (often called the white layer), a thin region of iron nitrides such as epsilon Fe2-3N and gamma-prime Fe4N, typically 2 to 25 micrometers thick. Beneath it lies the diffusion zone, usually 0.10 to 0.60 mm deep, in which fine precipitates of chromium, molybdenum, vanadium, and aluminum nitrides raise hardness progressively from the core toward the surface. On hot-work tool steels, surface hardness commonly reaches 900 to 1200 HV.

Because sleeve service life is directly tied to the integrity and depth of this case, nitriding layer depth verification is a mandatory release inspection in most die-casting tooling specifications. Insufficient depth leads to premature bore wear, galling, and seizure of the plunger; an excessively thick or brittle compound layer risks spalling and scoring of the melt-contact surface; and a non-uniform case produces uneven wear along the sleeve stroke. Layer depth measurement therefore confirms both process conformity and fitness for service.

• Confirms that the nitriding process met drawing requirements for case depth and hardness.
• Detects non-uniform nitriding caused by gas circulation, salt-bath depletion, or surface contamination.
• Controls compound layer thickness and porosity to prevent spalling in molten-aluminum contact.
• Provides objective evidence for first-article approval, batch release, and failure analysis.

Testing Samples

Cross-Sectioned Segments from Production Sleeves

The most representative sample is a ring or arc segment cut directly from a production sleeve, usually taken from a designated non-functional zone or from a sacrificial length allowance. The segment must be sectioned perpendicular to the nitrided surface using an abrasive cut-off wheel with abundant coolant to avoid burnishing or tempering the case. Samples should be marked to identify orientation, distinguishing the critical inner bore surface from the outer diameter and flange faces, because case depth frequently differs between these regions.

Witness Coupons Nitrided in the Same Batch

When destructive sectioning of a finished sleeve is not acceptable, witness coupons of the same steel heat, machined to convenient dimensions such as 15 to 20 mm diameter by 10 mm thick, are loaded into the nitriding furnace together with the sleeves. Coupons provide metallographic and hardness data without sacrificing the component, although it must be recognized that coupon surfaces may nitride slightly differently from the deep internal bore of a long sleeve, so coupon results are best used for statistical process control rather than absolute acceptance.

First-Article and Periodic Surveillance Samples

During process qualification and first-article inspection, multiple locations along the sleeve — both ends, mid-length, and several circumferential positions — should be evaluated to map case uniformity. For routine production, a defined sampling plan such as one sleeve per batch or per furnace load is sectioned and archived to maintain traceability of the heat-treatment process.

Testing Items

A complete nitriding inspection of a die-casting sleeve typically includes the following measurable characteristics, each reported independently:

• Total nitriding case depth, measured from the surface to the point where the nitrided microstructure is no longer distinguishable from the core.
• Effective nitriding hardening depth, defined by a hardness limit, typically the depth at which hardness falls to core hardness plus 50 HV.
• Compound layer thickness, including minimum, maximum, and mean values, and its continuity around the circumference.
• Compound layer porosity rating, since porous outer regions can act as crack initiation sites in service.
• Diffusion zone microstructure rating, including the presence of intergranular nitride networks that reduce toughness.
• Surface hardness and the microhardness gradient through the case.
• Core hardness verification to confirm the substrate was correctly pre-treated and tempered.
• Case uniformity between inner bore, outer diameter, end faces, and along the sleeve length.
• Checks for anomalies such as surface decarburization, oxide scale residues, or local soft spots.

Testing Methods

Two complementary laboratory methods are used: metallographic cross-section examination and instrumented microhardness traversal. Both require a properly prepared cross-section, and the microhardness method is generally considered the arbitration method whenever the metallographic estimate is disputed.

Specimen Preparation

Specimens are mounted in epoxy resin with edge-retention additives, ground through successive silicon-carbide grades, and polished with diamond suspensions to a finish suitable for microstructural evaluation, following the general practices of ASTM E3 and ASTM E407. Etching is performed with 2 to 4 percent nital for a few seconds, which darkens the diffusion zone and reveals the compound layer clearly at magnifications of 100× to 1000×. Care must be taken to avoid edge rounding or over-etching, since either error systematically biases compound layer and total depth measurements.

Metallographic (Microscopic) Method

Following the principles of ISO 18203 and GB/T 11354, the cross-section is examined at 100× to 500× to locate the boundary where nitriding-induced features — the compound layer, sub-surface nitride precipitates, and the etch-darkened diffusion zone — fade into the unaffected matrix. The distance from the outer surface to this boundary is recorded as the total case depth. Compound layer thickness is measured at a minimum of five to ten evenly spaced positions along a representative length of the bore surface at 500× or higher, and the mean, minimum, and maximum values are reported. The microstructure is also rated for porosity and nitride network severity in accordance with the rating charts of GB/T 11354.

Microhardness Traverse Method

The effective nitriding hardening depth is determined with a Vickers microhardness tester in accordance with ISO 6507-1 or GB/T 4340.1, using the microindentation practices of ASTM E384. A typical procedure is as follows:

• Select a test force of HV0.5 or HV1 for the traverse so that each indentation averages over many nitride precipitates and scatter is minimized; HV0.1 or lower forces are reserved for measuring the compound layer itself.
• Place the first indentation far enough from the surface to satisfy the ISO 6507-1 edge-distance rule of at least 2.5 indentation diagonals, then step inward or outward in increments that respect the minimum center-to-center spacing of three diagonals for steel — approximately 0.1 mm steps for HV0.5 on a 1000 HV case.
• Establish core hardness from at least three to five indentations well beyond the case, for example at depths greater than 2 mm or on the opposite wall of the section.
• Plot hardness versus distance from the surface, draw the limit line at core hardness plus 50 HV, and determine the intersection as the effective nitriding depth.
• Report the effective depth to the nearest 0.05 mm together with the hardness profile, the test force used, and the core hardness value.

Repeatability is improved by running two or three parallel traverses on the same cross-section and reporting the average with the observed spread. Measurement uncertainty for a competently performed traverse is typically on the order of 0.02 to 0.05 mm, and testing should be carried out by accredited third-party laboratories operating under ISO/IEC 17025 management systems whenever certification value is required.

Uniformity Assessment and Reporting

For die-casting sleeves, single-point results are not sufficient because gas flow and part geometry create local differences in case formation. A thorough report presents depth and hardness data for each tested location — inner bore at both ends and mid-length, outer diameter, and flange faces — identifies the minimum measured depth, and compares all values against the drawing tolerance. Photomicrographs of the etched cross-section at standardized magnifications should accompany the numerical results for traceability.

Standards and Acceptance Guidance

The inspection is governed by recognized national and international standards, including the following:

• ISO 18203, which defines methods for measuring the thickness of surface-hardened layers, including nitrided layers, by hardness and microstructure.
• GB/T 11354, which specifies the determination of nitriding layer depth and the metallographic structure inspection of steel parts, including porosity and nitride-network ratings.
• GB/T 9451, which covers the determination of the depth of thin surface-hardened layers by microhardness.
• ISO 6507-1 and GB/T 4340.1, which specify the Vickers hardness test method applicable to the microhardness traverse.
• ASTM E384, which standardizes microindentation hardness measurement practice.
• ASTM E3 and ASTM E407, which cover metallographic specimen preparation and microetching.

Acceptance criteria are always set by the component drawing, but typical industrial practice for die-casting shot sleeves nitrided in H13 calls for an effective depth of 0.30 to 0.50 mm, a continuous compound layer of approximately 5 to 15 micrometers with limited porosity, surface hardness of 950 to 1200 HV at low test forces, and no continuous intergranular nitride network at 500×. Ductile-iron sleeves are usually specified with shallower cases and correspondingly lower hardness. These values should be validated against the original equipment specification for each application.

Applications

Nitriding layer depth testing of die-casting sleeves supports quality assurance and engineering decisions across the entire tooling life cycle:

• Release inspection of new and re-nitrided shot sleeves, plunger bushings, and gooseneck liners by die-casting plants and heat-treatment providers.
• First-article qualification of new sleeve designs, new steel suppliers, or a change between gas, plasma, and salt-bath nitriding processes.
• Process monitoring of nitriding furnaces and salt baths through witness coupons evaluated on a statistical basis.
• Failure analysis of sleeves exhibiting premature bore wear, galling, plunger seizure, or compound-layer spalling.
• Research and development comparing nitriding parameters, steel grades, and compound-layer engineering for melt-contact tooling.
• Extension to related nitrided die-casting tooling such as ejector pins and sleeves, core pins, die inserts, and hydraulic cylinder components subjected to sliding wear.

FAQ

What is the difference between compound layer thickness and total case depth?

The compound layer is the thin, hard iron-nitride region at the very surface, normally a few micrometers to about 25 micrometers thick, while the total case depth extends from the surface through the entire diffusion zone and may reach 0.6 mm or more. Both must be measured and reported because they are controlled by different process variables and fail in different ways in service.

Which method is authoritative when metallographic and microhardness results disagree?

The microhardness traverse is the internationally recognized arbitration method because it defines depth by an objective hardness limit, typically core hardness plus 50 HV. The metallographic method remains essential for compound layer measurement and for rating microstructural features such as porosity and nitride networks.

Why are HV0.5 or HV1 forces used for the depth traverse instead of HV0.1?

Larger test forces produce larger indentations that average over many fine alloy-nitride precipitates, reducing scatter caused by individual particles. Smaller forces are used only for the compound layer or the near-surface region where a large indentation would interact with the edge or overlap the layer being measured.

Can nitriding depth be verified non-destructively?

There is currently no non-destructive technique that measures nitriding depth with certified accuracy on a sleeve bore. Eddy-current and magnetic methods can monitor process consistency, but calibrated depth values require a polished cross-section and either microscopy or microhardness indentation. This is why witness coupons nitrided in the same batch are widely used to preserve the component itself.

How many measurement positions are required for a reliable report?

Compound layer thickness should be measured at a minimum of five to ten evenly spaced positions per examined surface, and the depth traverse should be repeated on at least two profile lines. For full sleeve qualification, samples should be taken from both ends of the bore and from the mid-length region, reporting minimum and maximum values against the specification.

What process factors commonly cause shallow or non-uniform nitriding depth?

Frequent causes include poor gas circulation or gas dissociation in deep bores, exhausted or contaminated salt baths, residual oxide scale or passivated surfaces that block nitrogen uptake, insufficient removal of decarburized machining stock before nitriding, furnace temperature non-uniformity, and steel chemistry at the low end of nitride-forming elements such as chromium, aluminum, molybdenum, and vanadium. A well-designed depth inspection identifies which of these factors is responsible.