Nitrided Screw and Barrel: Steel Selection, Case Depth and Inspection

1. What is nitriding, and what does it change in a screw and barrel?

Nitriding is a thermochemical surface treatment that introduces nitrogen into steel to develop a hardened surface region. It changes the near-surface material; it does not add a separate barrel liner. A deposited nitride coating and a nitrided steel component therefore require different descriptions on a purchase order.

Identify the component and protected surfaces separately

Start with the actual part: screw, barrel, matched twin-screw set or another component. On a screw, identify the flight crests, flanks and channel surfaces covered by the treatment requirement. On a barrel, identify the working bore and any areas excluded from treatment. Drive connections, threads and locating faces need their own drawing instructions.

Put the gas nitriding process in its manufacturing context

EJS's nitrided screw barrel range identifies gas nitriding as its main current process. For a replacement order, confirm the route for the selected steel, its prior heat treatment, machining and surface preparation, nitriding, any subsequent finishing and final inspection. A process name or furnace cycle duration cannot replace the finished-part requirements.

This is the practical purpose of understanding the steel nitriding process: establish what condition enters each operation and what evidence must accompany the finished component. A comparison of gas and plasma methods is only useful if it leads to a verified route for the part being ordered.

2. Nitriding steel selection: confirm the grade and core condition

Read the grade together with the material record

EJS's single screw barrel product details list materials including 38CrMoAlA, 34CrAlNi7 and 31CrMoV9. The choice still needs to be tied to the component drawing, formulation and required finished condition. A list of available steels is not a ranking of which one will work best in every machine.

Check the named material standard, grade, heat number, chemical composition and supplied condition. Keep these records linked to the part or controlled production batch. When an international substitute is proposed, compare the actual specifications and acceptance requirements; a similar designation in a sales table is insufficient to approve the substitution.

Confirm the condition before nitriding

Steel producer Ovako describes 31CrMoV9 as intended for quenched and tempered parts that are generally machined and subsequently nitrided. That sequence matters: the steel grade and its prior condition are part of the surface-treatment specification.

Confirm the required core condition and compatibility of the proposed thermal sequence. Do not assume a surface treatment will correct an unsuitable core. The material record identifies the starting steel; the treatment and test records establish what happened afterward.

Nitriding carbon steel versus alloy nitriding steel

“Can this carbon steel be nitrided?” is a narrower question than “Will it meet this screw's requirements?” Plain-carbon and alloy steels can respond differently to treatment. Require evidence for the actual grade and condition rather than transferring a surface-hardness target from another steel.

For the broader distinction between base steel, surface treatment and working alloy, use the screw and barrel materials guide.

3. Is surface hardness enough to judge nitriding quality?

No. Surface hardness is one result at a particular location and test condition. It does not, by itself, show how hardness changes with depth. Nor does a reading at one end establish uniformity over the working length.

Keep the test method and force with the number

A report should identify whether the result is Vickers, Knoop, Rockwell or another method, together with the applicable force, location and preparation. Preserve the original measured scale. A converted HRC value is not a direct Rockwell test, and an HV1 designation carries test-force information that a bare “HV” value does not provide.

ISO 6507-1:2023 specifies the Vickers hardness test method. ASTM E384-22 describes microindentation testing for hardness variations over small distances, including surface-hardened regions. These methods help define a valid measurement; they do not choose the required nitrided depth for your screw.

Read the gradient, not only the peak

A cross-sectional microhardness traverse records values at successive distances from the surface. It can show where a specified threshold is crossed. Its usefulness depends on specimen preparation, indentation placement, the selected method and the stated evaluation rule.

For a finished curved surface or difficult-to-access bore, agree on a suitable examination method with the testing laboratory. Do not replace an agreed traverse with an easy-to-reach surface reading simply because both reports use the word “hardness.”

4. Case depth analysis: define the quantity before comparing results

Ask “depth to what?” before asking “how many millimeters?” Different reported depths can be valid descriptions of different features. Their names and evaluation criteria need to be explicit.

ISO 18203:2026 covers methods for determining surface-hardened layer depths, including nitriding hardness depth. Specify the applicable edition and evaluation requirements on the order. A shorthand such as ECD or TCD needs its definition too; terminology used in one laboratory's explanatory article should not silently replace the controlling specification.

Do 50 HRC, 513 HV and 550 HV mean the same requirement?

They should not be treated as interchangeable callouts. AHT's case-depth explanation illustrates a 50 HRC equivalent threshold as 513 HV and discusses depth evaluation from a hardness curve. It also distinguishes the conventions used for carburizing and nitriding. That example does not establish a universal 513 HV requirement for nitrided screws.

A 550 HV threshold is a different criterion. Read the process, standard edition, test force and agreed threshold on the actual drawing. Keep a hardness conversion separate from the laboratory's original measurement.

A worked example: the same profile, two reported depths

If I were reviewing a nitrided screw and barrel inspection package, I would align the depth criteria before comparing reports. Consider the following assumed hardness profile. It is a calculation example, not a production test.

Assume a core hardness of 320 HV and a project criterion of core hardness plus 50 HV. The resulting threshold is 370 HV. The example profile gives 390 HV at 0.40 mm and 350 HV at 0.50 mm. Linear interpolation between those points gives:

Now assume a second report evaluates the same profile at 500 HV. Its values are 540 HV at 0.20 mm and 460 HV at 0.30 mm. The same interpolation gives 0.25 mm.

I would not judge 0.45 mm as better than 0.25 mm from those numbers. The reports use different thresholds. I would retain the original profile, check the method, force, sampling location and finishing state, then review both against the same project requirement.

The result of this example is a comparison rule: two depth numbers can come from one hardness profile. Acceptance still depends on the drawing, sample representativeness and final dimensions.

5. Compound layer and final finishing: specify the condition you will receive

Give the compound layer its own requirement

The compound layer, often called the white layer in metallographic examination, is not a substitute name for the complete nitrided case. If its thickness, condition or removal matters to the working surface, specify that examination separately. AHT's sectioned nitrided-steel example identifies a compound zone and an underlying diffusion zone.

A purchase order should establish whether the layer is retained, limited or removed for the intended duty. Avoid imposing one blanket white-layer rule on every screw and barrel. The required condition needs to follow the material, surface function and agreed treatment route.

Measure from the agreed final surface

If grinding, polishing or honing follows nitriding, identify the material-removal allowance and the state represented by each report. A depth measured before finishing does not automatically establish the residual condition after finishing.

Record final screw diameters and barrel-bore dimensions against the approved drawing. Include the relevant straightness, locating and interface requirements. “Low distortion” is not a reason to omit final dimensional inspection, and a satisfactory hardness-depth result does not prove the screw-to-barrel clearance.

6. Where should a screw and barrel be inspected?

Map working faces and bore positions

Mark the required locations on a drawing with a clear axial reference. For the screw, identify the working zones and surfaces covered by the plan. For the barrel, identify the axial and circumferential bore positions to be represented. Use the component's actual geometry rather than assuming every screw has the same functional sections.

A reading on an outside flange cannot automatically establish the condition of a deep internal working surface. Likewise, a test near the drive end cannot establish the result throughout a screw's working length. The inspection plan must explain how the selected locations address the requirement.

Agree on sectioning and coupon evidence before production

A cross-sectional traverse generally requires a prepared section. Decide before manufacture whether the evidence will come from an agreed component section, extension, witness coupon or another controlled sampling arrangement. A receiving inspector should not have to invent a destructive sampling plan after the finished parts arrive.

For a witness coupon, record its steel, prior treatment, surface preparation, furnace batch and relevant loading conditions. Consider whether it represents the actual working surface and geometry. Sharing a furnace batch is useful traceability, but is not, by itself, evidence that every location in a long bore has the same result.

Factory perspective: make the report answer the drawing

When reviewing a nitrided screw and barrel order, I first separate the screw from the barrel, then check the requirements for each working surface. A quotation promising high hardness and a deep case is not enough to arrange acceptance. I would confirm the steel, core condition and drawing revision, then ask which depth criterion applies, where it will be measured and whether the report represents the treated or final-finished state.

If a report provides only one surface-hardness value, I would ask for the method, force and location. A reading at the screw end cannot automatically represent the entire working length, and a result on the barrel flange cannot replace bore evidence. When a sample is used for sectional testing, I would also check its material, prior condition and treatment batch to establish what it represents.

Finally, I would put the hardness profile, depth result and final dimensions on the same review sheet. The purpose is to give purchasing, production and inspection one agreed basis for judging the delivered parts.

7. When should you reconsider a nitrided specification?

Reopen the material review when the formulation, feedstock source or observed damage changes. Record filler type and loading, fibers, pigments, additives, recycled-feed variability and the location of damage on the old components. “PP” or “PVC” alone is not a complete operating description.

Do not assume a higher hardness or a deeper case will correct local contact, unsuitable geometry or a fit problem. Investigate the damage and machine condition before attributing every mark to insufficient nitriding. Material selection and dimensional compatibility must be reviewed together.

If the working-surface system itself needs reassessment, use the bimetallic versus nitrided screw barrel comparison for that broader decision. Specify the screw and barrel individually, then verify the pair. There is no need to invent a universal filler percentage, operating-hour threshold or service-life multiplier to make the decision useful.

8. What should the inspection report contain before shipment?

The report should show actual results and their relationship to the ordered part. A specification sheet stating target values is not a record of what was measured.

When a result is missing its method, position or manufacturing state, ask for the original record or an agreed supplementary examination. If two reports use different depth criteria, reconcile the criteria before deciding which result meets the order. Keep any unresolved item visible in the release decision.

DIN lists DIN 50190-3:1979-03 as withdrawn, with DIN EN ISO 18203:2022-07 as its replacement. The international ISO document now has a 2026 edition. Preserve the standard and edition specified by the controlling drawing, and agree any substitution through the project's document-control process. A withdrawn standard number does not automatically invalidate an older contractual requirement.

The wider order-to-arrival checks are covered in EJS's screw barrel quality inspection guide.

9. Send a drawing and inspection package with your inquiry

A useful inquiry identifies both the part to be made and the evidence needed to accept it. Send the original-machine information, component drawing and revision, formulation, old-part measurements or damage photographs, and the reason for replacement.

• Name the proposed steel and any controlled substitution requirements.
• Mark the working surfaces, excluded areas and final dimensional requirements.
• State the treatment route where it is a project requirement.
• Identify the hardness method, depth definition and applicable standard edition.
• Agree the sampling arrangement and the condition represented by the results.
• List the material, treatment and inspection records required for release.

EJS's single-screw product information states that material certificates and inspection reports accompany supplied screw barrels. Confirm the scope needed for your order, particularly any sectional examination, witness samples or additional laboratory work. Those requirements should be resolved before production rather than assumed from a general statement about inspection.

10. Frequently asked questions