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How Cold Forged Parts Are Inspected
How Cold Forged Parts Are Inspected: Hardness, Tensile, CMM and Cpk
Cold forged parts are inspected at several stages using dimensional checks, hardness and material tests, surface inspection and statistical process control. The goal is not just to catch bad parts at the end, but to prove the process stays stable so bad parts are not made in the first place.
This guide explains the main inspection methods, what each one tells you, and what buyers should ask a supplier.
Inspection Happens at Three Stages
| Stage | What Is Checked | Why |
|---|---|---|
| Incoming | Raw material: grade, certificate, dimensions, surface condition | A part can't be better than its steel |
| In-process | First piece, periodic samples during the run, tooling condition | Catch drift before it makes scrap |
| Final | Finished dimensions, hardness, appearance, coating, packing | Confirm the part meets the drawing before shipping |
Good suppliers weight the effort towards the first two. Final inspection alone only sorts good from bad. It doesn’t prevent bad.
Incoming Material Checks
Before forging starts, the steel is verified:
- Material certificate (mill test certificate) matching the ordered grade, such as SWRCH18A or 16MnCr5. See our steel grade guide.
- Dimensions of wire or bar, such as diameter and straightness
- Surface quality, since seams or cracks on the wire can open up during forming
- Hardness or microstructure checks where the process needs a soft, annealed starting condition
- Lot identification, so any part can be traced to its steel
Dimensional Inspection
Dimensional checks confirm the part matches the drawing.
Common tools
- Vernier calipers and micrometers for basic diameters and lengths
- Go / no-go gauges and plug or ring gauges for fast checks of bores, diameters and threads in production
- Height gauges and surface plates for heights and flatness
- Profile projectors and optical comparators for outlines and small features
- Coordinate measuring machines (CMM) for complex shapes, geometric tolerances and full reports
CMMs measure points in three dimensions and compare them with the design. They are especially useful for checking features such as concentricity, perpendicularity and position that simple gauges can’t capture.
Gauge control matters as much as measurement. Gauges must be calibrated to traceable standards, and the measurement method itself must be reliable. This is checked through measurement system analysis (MSA), including gauge repeatability and reproducibility studies. See the PPAP guide for where this fits.
Hardness Testing
Hardness is one of the quickest ways to confirm heat treatment and material condition.
- Rockwell is the most common in production, quick and direct to read.
- Vickers uses a small diamond indent, suited to thin sections and to measuring hardness at specific depths.
- Brinell uses a larger indent, suitable for coarser or softer material.
Case depth for case-hardened parts. For carburised parts such as bearing cups and races, surface hardness alone is not enough. The hardened layer’s depth is measured on a cut section with a microhardness traverse, so the case depth meets the drawing. See our heat treatment page for the process.
Tensile and Strength Testing
Tensile testing pulls a specimen or finished part until it deforms or breaks, measuring its strength under load.
- It confirms strength (and, on specimens, elongation) against the specification.
- On assemblies, such as welded parts, it verifies joint integrity.
Our stabilizer link manufacturing guide shows how specialised tensile testing and a Universal Testing Machine are used on a real product.
Metallurgical Checks
Cutting, polishing and etching a sample part reveals what the surface can’t:
- Grain flow. Cold forging makes the grain follow the shape of the part, which supports strength and fatigue life. A cut-and-etched section can confirm it.
- Microstructure. Checks that heat treatment gave the intended structure.
- Internal defects, such as laps or voids.
- Decarburisation at the surface, which can soften a part.
These tests are usually done at set intervals or when something in the process changes.
Surface and Crack Inspection
- Visual inspection against agreed standards and boundary samples
- Magnetic particle inspection to find surface and near-surface cracks in magnetic steels
- Eddy current testing for certain high-volume, safety-critical parts
- Surface roughness measurement where a finish is specified
- Coating checks after phosphating, such as coating uniformity and weight. See zinc vs manganese phosphating.
Safety-critical parts such as steering and suspension joints often get stronger crack-detection requirements.
Statistical Process Control and Cpk
Measuring every part is slow, and it still doesn’t tell you whether the process is healthy. Statistical process control (SPC) does:
- Control charts plot measurements over time. A trend or sudden shift shows a problem (tool wear, material change) before parts go out of tolerance.
- Process capability (Cpk, Ppk) compares how much the process varies with how much room the tolerance allows.
Reading Cpk in plain terms
- A high Cpk means the process sits comfortably inside the tolerance with margin to spare.
- A low Cpk means the process is close to the limits, and some parts will fall outside.
- Automotive customers commonly expect about 1.33 or higher for ongoing production on key characteristics, and more for critical ones. The customer’s own requirement always applies.
Capability is typically studied on the characteristics marked critical on the drawing, not every dimension.
Sampling and Traceability
- First-piece approval at the start of every run or after any setup change
- Periodic in-process sampling at set frequencies from the control plan
- Lot sampling at final inspection, based on an agreed plan
- Batch and lot traceability, so any part links back to its steel lot, process settings and inspection records
If a problem is found later, traceability decides whether you recall one lot or everything.
What Buyers Should Ask a Supplier
- What is checked at incoming, in-process and final stages?
- Which gauges and instruments do you use for my critical dimensions?
- How are gauges calibrated and how is the measurement system verified?
- Do you track Cpk on key characteristics, and can you show an example?
- How do you test hardness and case depth on heat-treated parts?
- What crack-detection methods do you use on safety-critical parts?
- How is each lot traced to its material and inspection records?
- What happens when a part fails, and how are suspect parts contained?
Clear, specific answers, backed by sample reports, are a strong sign of a controlled process.
Inspection at ABC
Cold Forging: Ideal for mass production with high repeatability
Accurate Bearing Components, in Doddaballapur near Bangalore, has made precision automotive components since 1992. Our quality system is audited to IATF 16949 and ISO 9001, and we hold MSME ZED Gold certification. We use specialised tensile testing equipment, including a Universal Tensile Testing Machine, for strength verification where parts require it. Inspection is tied to our process route across cold forging, CNC turning, heat treatment and phosphate coating.
To discuss inspection and documentation requirements for your part, send your drawing to ops@accuratebearing.in or call +91 95357 42213. Our RFQ checklist shows what to include, and our cold forged components page lists the parts we make.
Hot Forging: Better suited for low-volume or large, complex components
Table of Contents
FAQs
Through incoming material checks, in-process sampling and final inspection. Methods include gauges, calipers, micrometers and CMM for dimensions, hardness testing, tensile testing, metallurgical checks and crack detection.
Cpk measures how well a process stays within the tolerance. A higher value means more margin and fewer out-of-tolerance parts. Automotive customers often expect about 1.33 or higher on key characteristics.
A coordinate measuring machine measures parts in three dimensions and compares them to the design. It is used for complex shapes and geometric tolerances that simple gauges cannot check.
Usually with Rockwell testing for speed, or Vickers for thin sections and depth profiles. Case-hardened parts also need case depth measured on a cut section.
Visual inspection and magnetic particle inspection are common. Eddy current testing is used on some safety-critical parts.
It links every part to its steel lot, process data and inspection records, so if an issue appears, only the affected lot needs action.
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