Non-destructive testing
Not all test methods that are common for carbon steel work on austenitic stainless steel welds. Magnetic particle testing is ruled out, and ultrasonic testing is only of limited use. For surface cracks, penetrant testing is the method of choice.
| Method | Used for | Limits on stainless steel | Standard |
|---|---|---|---|
| Visual testing (VT) | always, as the first test | weld shape, surface imperfections, temper colours | ISO 17637 |
| Penetrant testing (PT) | surface cracks on all materials | only imperfections open to the surface | ISO 3452-1 |
| Magnetic particle testing (MT) | only ferritic and martensitic grades | austenitic steels cannot be magnetised and therefore cannot be tested | ISO 17638 |
| Eddy current testing (ET) | surface imperfections on conductive materials | requires suitable probes and experience | ISO 17643 |
| Radiographic testing (RT) | pores, cavities, undercuts, lack of fusion in the beam direction | planar imperfections transverse to the beam hardly visible, no depth information from a single exposure | ISO 17636 |
| Ultrasonic testing (UT) | internal imperfections, including their depth | considerably limited on stainless steel: the grain structure of the weld metal scatters the sound and simulates imperfections; only from 6 mm wall thickness | ISO 17640 |
Based on DVS 0931, section 10; standards brought up to date. The method and extent of testing depend on the loading of the component and the applicable code. NDT personnel must be qualified to ISO 9712.
Which imperfections are permissible is governed by the quality levels of ISO 5817. Causes of typical defects: Weld defects and corrosion damage.
What is additionally tested on stainless steel
Besides cracks and pores, the microstructure and surface determine the corrosion resistance of stainless steel. There are dedicated tests for these.
| Characteristic | How | Standard | |
|---|---|---|---|
| Ferrite content | magnetically with a ferrite meter, result in FN | ISO 8249 | Microstructure, Duplex |
| Residual oxygen during purging | residual oxygen meter close to the root | DVS 0937 | Root protection |
| Temper colours | comparison with colour charts or reference samples | by agreement | Temper colours |
| Resistance to pitting corrosion | critical pitting temperature (CPT) in the ferric chloride test | ASTM G48 | Duplex |
| Resistance to intergranular corrosion | intergranular corrosion test | ISO 3651-2 | Basics |
| Interpass temperature | contact thermometer or temperature indicating crayon | ISO 13916 | Heat control |
Which of these tests are required is specified by the order, the applicable code or the welding procedure specification.
Quality assurance
The quality requirements for fusion welding are set out in ISO 3834, the tasks of welding coordination in ISO 14731.
- Welding procedure specification (WPS) to ISO 15609-1, qualified by a procedure test to ISO 15614-1. The purging procedure belongs in it as well.
- Welder qualification to ISO 9606-1.
- Approved wire-gas combination for welds subject to acceptance; for construction products subject to building regulations, only approved filler metals and consumables.
- Material certificates: For the choice of filler metal and the Schaeffler diagram, inspection certificates 3.1 to EN 10204 are the best basis.
Health and safety during welding
The general rules apply when welding stainless steel, plus three special features: fume containing chromium and nickel, highly reflective surfaces, and the handling of forming gas and pickling acid.
Welding fume
Besides iron, the fume contains chromium, nickel and manganese and their compounds, including carcinogenic chromium(VI) compounds. It should be extracted as close to the source as possible; the limit values of TRGS 900 must be observed. The relevant rules are TRGS 528 “Schweißtechnische Arbeiten” (welding work) and ISO 15011. Outside Germany, the corresponding national regulations apply.
Radiation
For the MAG arc, protective filters of at least shade 11 are required. Stainless steel surfaces reflect UV radiation particularly strongly, so even parts of the body in apparent shadow can be affected. Additional protective clothing, partitions or curtains protect welders and bystanders.
Cleaning agents
No chlorinated solvents such as perchloroethylene or trichloroethylene: under the UV radiation of the arc, they form the highly toxic gas phosgene. Acetone is suitable for degreasing.
Forming gas and pickling acid
Two hazards are typical of stainless steel fabrication: asphyxiation and explosion during purging, and chemical burns during pickling.
- Asphyxiation: Backing gases are odourless, colourless and tasteless. In confined spaces and large purged vessels, oxygen deficiency can occur without warning. Ventilate vessels slowly and vent the exhaust gas to the open air.
- Ignitable mixtures: Hydrogen-containing gases above about 5.5 % H₂ (forming gas) or 4.2 % H₂ (argon-hydrogen) can ignite with air. Purge with nitrogen or argon before and after, do not flare off. Details in Root protection and purging.
- Pickling acid: Pickling agents usually contain nitric and hydrofluoric acid. Only trained personnel with protective clothing, safety goggles, respiratory protection where necessary and occupational health surveillance. Do not discharge rinsing water untreated; pickling sludge is hazardous waste. More in Temper colours, pickling, passivation.
- Grinding dust should be removed straight after the work.
Frequently asked questions
Can I test stainless steel welds with magnetic particles?
Not austenitic welds, because they cannot be magnetised. For surface cracks, penetrant testing is used. With ferritic and duplex steels, magnetic particle testing is possible, but because of the two-phase microstructure it is rarely used on duplex.
Why is ultrasonic testing difficult on stainless steel?
The coarse, oriented microstructure of austenitic weld metal scatters and reflects the sound. This simulates imperfections and makes evaluation more difficult.
Is welding fume from stainless steel more hazardous than from carbon steel?
Yes, because of the chromium and nickel compounds. That is why extraction right at the source is especially important.
Further reading: articles on welding stainless steel
The series from the material via process, gas and root protection to the tested weld. For an overview, see Welding of stainless steel.
- What makes stainless steel stainless
- Microstructure, delta ferrite and hot cracking
- Choosing the filler metal
- Processes at a glance
- Shielding gases
- Heat input and interpass temperature
- Joint preparation and tack welding
- Root protection and purging
- Temper colours, pickling, passivation
- Duplex and super duplex
- Joining stainless to carbon steel
- Weld defects and corrosion damage
About the author
Robert Lahnsteiner, Ing., is an International Welding Engineer (IWE) and managing director of MIGAL.CO GmbH. After training in telecommunications and electronics at HTL Braunau, he spent eighteen years in welding technology at Fronius from 1983. Since 2001 he has run MIGAL.CO, the aluminium welding wire manufacturer in Landau an der Isar, together with Martin Erl. His focus is aluminium filler metals, wire manufacturing and wire feeding, and the monitoring of welding processes. Within DVS, the German Welding Society, he has served for around 30 years on technical committee FA03 “Arc welding” and chairs working group 2.4.4 “MIG welding of aluminium”.