How to Identify Metal: 10 Field Tests Every Welder Should Know
Knowing how to identify metal before you weld saves cracked joints, rusted stainless and hazardous fume, because every metal needs a different process, filler and heat control. This guide gives the field tests welders and inspectors actually use, from a magnet to a handheld analyser.
Quick Answer: To identify metal, test in stages: read markings and colour, check with a magnet, compare weight or density, run a spark test, then confirm with a hardness check or chemical spot test. A magnet separates steel from aluminium and copper, but only positive material identification (PMI) with an XRF or OES analyser confirms the exact grade for critical work.
You will learn what each test can and cannot prove, how to read a spark test, how to calculate density, and when PMI is required. Properties referred to here are explained in the mechanical properties of metals guide, and the P-Number guide shows how identified metals are grouped for welding procedures.
Field tests sort metals into families. Where a grade matters, the certificate and PMI decide.
Key Takeaways
- Test in stages: look, magnet, density, spark test, hardness or spot test, then PMI for critical alloys.
- A magnet separates steel and cast iron from aluminium, copper and brass, but cannot give a grade; cast iron is magnetic.
- Density separates light metals and copper alloys from steel, yet carbon steel and 304 stainless differ by only about 1 percent.
- A spark test sorts steel families by spark colour and bursts, but it is operator dependent and can contaminate stainless.
- Only PMI by XRF or OES verifies an exact grade, and XRF cannot measure carbon, so use OES for L grades.
What Is Metal Identification and Why Does It Matter for Welders?
Metal identification is the process of determining the type and grade of an unknown metal using visual, physical and chemical tests, from magnets to spectrometers. For welders and inspectors the goal is practical: decide whether a part is steel, stainless, aluminium, copper alloy or cast iron before choosing a process, filler and heat treatment. Test results build on the properties described in the mechanical properties of metals guide, and the terms are defined in the welding terminology A-Z glossary.
How do you identify what type of metal something is?
Identify an unknown metal in stages: note markings, colour and finish; test with a magnet; compare weight or measure density; run a spark test on a grinder; then confirm with a hardness check or spot test. These stages narrow the family, but only PMI by XRF or OES confirms an exact grade for code or critical service.
Why it matters. Welding the wrong metal causes cracking, lost corrosion resistance or toxic fume. Stainless contaminated with carbon steel rusts, and cast iron welded like steel cracks.
Step 1: What Can You Learn by Looking First?
Appearance gives the first clues, so clean a small spot with a file or flap disc and look before testing. Colour separates the copper family quickly: copper is red-orange, brass is yellow, and bronze is a duller gold-brown. Aluminium is silver-white and soft, zinc and lead are dull blue-grey, and mill-scaled steel is blue-black.
- Markings: stamped grades, heat numbers and paint codes link a part to its material test certificate.
- Surface: rough sand-cast skin suggests castings, while smooth rolled finish suggests plate, pipe or sheet.
- Rust: orange rust means iron or steel; white powder on a light part suggests aluminium or zinc.
Treat markings as a lead, not proof. Rely on them only when backed by the certificate, as covered in mechanical testing of materials for properties reported on mill certificates.
Step 2: What Does the Magnet Test Tell You?
The magnet test sorts metals into magnetic and non-magnetic families in seconds, and it is the best first test. Use a small, strong magnet on a clean flat spot.
| Metal | Magnet response | Notes |
|---|---|---|
| Carbon and low-alloy steel | Strongly magnetic | Includes mild steel and Cr-Mo steels |
| Cast iron | Magnetic | Often slightly weaker pull than steel |
| Ferritic and martensitic stainless (400 series) | Magnetic | Cannot be separated from steel by magnet alone |
| Duplex stainless | Magnetic | Roughly half ferrite; see the duplex stainless steel guide |
| Austenitic stainless (304, 316) | Normally non-magnetic | Cold work and welds can make it weakly magnetic |
| Nickel | Magnetic | Monel 400 is often weakly magnetic; Inconel 625 is non-magnetic |
| Aluminium, copper, brass, bronze, zinc, lead, titanium | Non-magnetic | Separate by weight, colour and hardness |
Common myth. Some sources say cast iron is not magnetic. It is. A magnet separates ferrous metals from non-ferrous ones, but not cast iron from steel.
Step 3: How Do You Identify Metal by Weight and Density?
Density is a physical constant, so it separates look-alike metals that colour and magnet cannot. Lift the part against a known sample of the same size, or measure density by weighing in air and in water.
m_air = mass in air (g), m_water = apparent mass fully submerged (g), water = 1.00 g/cm3 Worked example: m_air = 270.0 g, m_water = 170.0 gdensity = 270.0 / (270.0 – 170.0)
density = 270.0 / 100.0 = 2.70 g/cm3, which matches aluminium
Density cannot separate carbon steel (7.85) from 304 stainless (about 7.9), because the difference is about 1 percent. Use it to sort light metals, brass and copper from steel.
Step 4: How Does a Spark Test Identify Steel?
A spark test identifies steel families by touching the sample lightly to a grinding wheel and reading the spark stream. Spark colour, length and bursts change with carbon and alloy content.
| Metal | Typical spark stream |
|---|---|
| Low-carbon steel | Long straw-yellow streaks with few bursts |
| Medium and high-carbon steel | Bright yellow-white with many fine bursts that increase with carbon |
| Cast iron | Short red-orange streaks with small repeating sparklers |
| Austenitic stainless | Short, dull orange streaks with few bursts |
| Aluminium, copper, brass | No usable sparks; the wheel loads with metal |
- Technique: angle the spark stream to one side against a dark background and use light, constant pressure.
- Safety: wear eye and face protection, keep the guard on, and keep sparks away from flammable material.
- Limit: the test depends on operator skill, and it can contaminate stainless steel and nickel alloys with iron from the wheel. Use a dedicated wheel and test scrap, not the finished surface.
What Other Quick Tests Identify Metal?
Several low-cost checks add evidence after the magnet and weight tests. None of them proves a grade, but each narrows the candidates.
| Test | What to observe | Identifies |
|---|---|---|
| File or scratch | Hardened steel skates over the file; aluminium, copper and lead cut easily | Soft versus hard metals |
| Chip test | Cast iron chips crumble and break; steel gives continuous curled chips | Cast iron versus steel |
| Ring or sound test | Cast iron gives a dull thud; steel rings; aluminium sounds duller | Cast iron versus steel |
| Copper sulphate spot | Copper plates onto clean mild steel but not onto stainless in a short test | Carbon steel versus stainless |
| Molybdenum spot-test kit | Colour change indicates molybdenum | 316 versus 304 |
| Eddy current sorting | Conductivity and permeability differ by alloy | Sorting known alloys |
Spot-test kits use chemicals, so wear gloves and eye protection and follow the supplier safety data sheet. Eddy current methods are covered in the ASME Section V overview. ASTM E1476 documents metals identification, grade verification and sorting methods.
How Does Hardness Testing Help Identify Metal?
Hardness gives an indirect estimate of strength and separates soft and hard grades of the same family. Portable Leeb or ultrasonic contact impedance testers work on site, and the methods are explained in the mechanical testing of materials guide.
An estimate near 495 MPa is consistent with common structural carbon steel, but it does not prove the grade. Use it to flag material that is clearly harder, softer or higher-carbon than expected, and check carbon equivalent before welding hard steel.
How to Identify Metal Step by Step
A reliable field workflow starts with the cheapest test and adds evidence only as needed. Follow these seven steps in order, then use the flowchart below.
- Clean and look. Remove paint, rust and scale from a small area, then read markings, heat numbers, stamps and colour.
- Run the magnet test. Hold a magnet to the clean spot and note whether it sticks strongly, weakly or not at all.
- Compare weight or density. Lift the part against a known sample, or weigh it in air and in water to calculate density.
- Spark test the magnetic metals. Touch the sample lightly to a grinder and compare spark colour, length and bursts with the chart.
- Check hardness or chip behaviour. Use a file, a chisel chip or a portable hardness tester to separate close candidates.
- Apply a spot test where needed. Use a controlled spot-test kit, following the supplier safety data sheet, to separate stainless grades.
- Confirm with PMI. Verify critical alloys with XRF or OES and record the result against the material test certificate.
When Do You Need PMI to Identify Metal?
Positive material identification (PMI) confirms the alloy grade by measuring chemical composition, using a handheld X-ray fluorescence (XRF) or optical emission spectrometry (OES) analyser. PMI does not replace the material test certificate; it checks the actual piece against it. Where PMI sits in inspection planning is shown in the ITP preparation guide.
| Method | What it tells you | Reliability for grade | Notes |
|---|---|---|---|
| Visual and markings | Likely family | Low | Needs certificate to support it |
| Magnet | Magnetic or not | Low | Fast first screen |
| Density | Metal family | Medium | Cannot separate steel and stainless |
| Spark test | Carbon steel family | Low to medium | Operator dependent |
| Hardness | Strength estimate | Medium | Indirect |
| Handheld XRF | Alloy elements | High | Cannot measure carbon |
| Handheld OES | Alloy elements and carbon | High | Leaves a small burn mark |
| Laboratory analysis | Full chemistry | Highest | Destructive, slower |
XRF cannot confirm L grades such as 316L, because carbon is a light element, so use OES when carbon content is critical. Handheld XRF contains an X-ray source, so only trained and authorised operators should use it, under the local radiation regulator. Owner specifications often require verified items to be marked “PMI” with a low-stress stamp, and the report to record method, instrument, calibration and result. ASTM E1476 and API RP 578 give guidance, and ASTM E415 covers spark OES analysis of carbon and low-alloy steel.
Regional note. On Indian and Gulf projects, mixed stock in yards and humid conditions that hide colour under rust make field identification less reliable. Contracts commonly require 100 percent PMI on alloy piping, so do not rely on spark or magnet checks for acceptance.
What Should You Do Once the Metal Is Identified?
After identification, match the process, filler and precautions to the metal, and classify the material by P-Number for procedure selection using the P-Number guide. Table 5 gives typical starting points; the WPS governs.
| Metal | Typical consideration |
|---|---|
| Low-carbon steel | Weldable with common processes; low-hydrogen consumables are typical for structural work |
| Medium and high-carbon steel | Hardens in the heat-affected zone; control preheat and check carbon equivalent |
| Cast iron | Brittle and high-carbon; repair welding only, with preheat, slow cooling and nickel-based consumables |
| 304 and 316 stainless | Austenitic filler, low heat input; see stainless steel weld decay |
| Duplex stainless | Matched duplex filler and controlled heat input |
| Aluminium | Remove oxide first; consult the guide to welding stainless, aluminium and copper alloys |
| Brass, galvanised steel | Zinc fume: remove coating at the joint and ventilate |
Once the grade is known, select the filler using the welding consumable selection guide and the TIG filler rod selection guide. The effect of alloying elements on weld behaviour is explained in alloying elements in steel.
Common Mistakes and Limits of Field Identification
Field identification has real limits, and requirements depend on the code edition and contract specification applicable to your project.
- Treating a magnet result as a grade: both 304 and aluminium are non-magnetic.
- Assuming cast iron is non-magnetic.
- Choosing consumables from a spark test on alloy or stainless steel.
- Grinding stainless with a carbon-steel wheel and contaminating the surface.
- Skipping PMI on alloy piping because the paint code looked right.
Limit. Field tests sort families. Only PMI and the material test certificate verify a grade for code, pressure or corrosion-critical work.
Quick Reference: Metal Identification Cheat Sheet
| Clue | Likely metal |
|---|---|
| Strong magnet, long yellow sparks | Carbon or alloy steel |
| Magnet, short red-orange sparks, brittle chips | Cast iron |
| Non-magnetic, very light, silver-white | Aluminium (or magnesium, titanium) |
| Non-magnetic, red-orange | Copper |
| Non-magnetic, yellow, heavy | Brass or bronze |
| Non-magnetic, heavy, silver-grey, no sparks of note | 300 series stainless or nickel alloy |
| Magnetic but stainless-looking | 400 series or duplex stainless |
| Any critical alloy | Confirm with PMI |
Frequently Asked Questions
Can a magnet tell stainless steel from carbon steel?
Is cast iron magnetic?
How do I tell aluminium from stainless steel without tools?
How accurate is a spark test?
Can XRF measure carbon in steel?
Is it safe to weld metal I cannot identify?
What is the difference between PMI and a mill test certificate?
Key Terms
- PMI
- Positive material identification, a check of alloy composition on the actual component.
- XRF
- X-ray fluorescence, a non-destructive analyser that cannot measure carbon.
- OES
- Optical emission spectrometry, a spark-based analyser that can measure carbon.
- Ferrous metal
- A metal containing iron as the main element, such as steel and cast iron.
- Ferromagnetic
- Strongly attracted to a magnet, as in carbon steel, cast iron and nickel.
- Density
- Mass per unit volume, usually in g/cm3 for metals.
- Heat number
- The mill heat identifier that links a part to its test certificate.
Conclusion
To identify metal, work from cheap tests to expensive ones: clean and look, magnet, density, spark test, hardness and spot test, then PMI for anything critical. A magnet sorts ferrous from non-ferrous metals, density separates light and heavy families, and a spark test sorts steels, but none gives a grade by itself. Record the result against the certificate and select the welding procedure from the confirmed material. As a next step, review the mechanical testing guide to understand the properties your certificate reports.
About This Guide
This guide was prepared by the WeldFabWorld technical team from the standards listed in References. Verify all requirements against the code edition and specification applicable to your project.
Standards and References
- ASTM E1476 – Standard Guide for Metals Identification, Grade Verification, and Sorting. ASTM International.
- ASTM E415 – Standard Test Method for Analysis of Carbon and Low-Alloy Steel by Spark Atomic Emission Spectrometry. ASTM International.
- ASTM E527 – Standard Practice for Numbering Metals and Alloys in the Unified Numbering System (UNS). ASTM International.
- ASTM A956 – Standard Test Method for Leeb Hardness Testing of Steel Products. ASTM International.
- ASTM A1038 – Standard Practice for Portable Hardness Testing by the Ultrasonic Contact Impedance Method. ASTM International.
- API RP 578 – Material Verification Program for New and Existing Alloy Piping Systems. American Petroleum Institute (API).
- ASME BPVC Section II – Materials. American Society of Mechanical Engineers (ASME).
- ASME BPVC Section IX – Welding, Brazing, and Fusing Qualifications. American Society of Mechanical Engineers (ASME).
- EN 10204 – Metallic products – Types of inspection documents. European Committee for Standardization (CEN).
Official sources: ASTM International, ASME and American Petroleum Institute.