The conclusive methods for determining the grade of a sample of metal, believed to be a stainless steel is by a chemicalanalysis method. Normally spectrographic methods are used. These quantitative methods, provided they are properly calibrated using samples of known composition, give accurate figures for the important elements (chromium, nickel, molybdenum) in the sample.
In most cases this enables a stainless steel grade to be assigned to the sample. Additional analysis may still be needed to differentiate between low carbon (0.03% maximum) and ‘normal’ carbon (0.06/0.07%) variants eg 304L (1.4307) and 304(1.4301) or for detecting nitrogen additions in the steel. Without specialist analysis equipment differentiating between stainless steel grades or checking if a sample is a stainless steel rather than a low alloy or carbon steel type, is difficult.
This article attempts to provide some guidance on non-laboratory detection methods, firstly looking at physical, mechanical and corrosion resisting properties of metals. Often a combination of tests will be needed to reach any sort of meaningful conclusion on the steel type. Finally, a summary step-by-step procedure is suggested.
Metals such as copper and gold and their alloys (eg brasses & bronzes) are easily distinguishable from other metals. To the untrained eye however trying to differentiate between most other metals is not practical. It may be possible to differentiate between polished pieces of austenitic (eg 1.4301, 304) and ferritic (eg 1.4016, 430) on colour. The austenitic has a yellow tinge, the ferritic a blue, more ‘metallic’ tinge. A magnet test is however more conclusive.
Most ferrous alloys (ie steel and cast irons) have similar densities and even laboratory methods would not be able to distinguish between carbon steel and stainless steel. Away from the laboratory only significant differences in density would give any clues in sorting metals. Heavier (more dense) metals like lead or tungsten or lighter metals like aluminium or magnesium may be easy to recognise from their relative weights. Metals with densities closer to those of iron, eg nickel, chromium and zinc are very unlikely to be distinguishable from their relative weights.
Iron alloys, provided they are free of gross internal defects, usually have a characteristic metallic ring ie like a bell, when struck or dropped onto a hard surface. Some metals, notably, lead and aluminium have a ‘duller’ sound, if tested in the same way.
Iron, nickel and cobalt are ferromagnetic at normal (ambient) temperatures. This means that they strongly attracted to a permanent magnet. Most iron alloys are also ferromagnetic, including grades in the ferritic, martensitic and duplex stainless steel families. Softened austenitic stainless steel however are not ferromagnetic and so are not attracted to a permanent magnet. This can provide a basis for sorting between softened austenitics and other stainless and non-stainless steels. If austenitic stainless steel are cold worked, they can behave as “partially” ferromagnetic, showing some attraction to a permanent magnet. With complex shape formed components the partial magnetic attraction is usually non-uniform and is more marked at formed corners or near drilled holes or machined faces. This uneven distribution is often useful in confirming the steel as an austenitic type. This variation in attraction to a magnetic does not occur with other stainless steel, carbon steel or metals like aluminium.
Grinding sparks (frictional sparking)
Grinding sparks have been used traditionally in metal manufacturing industries as a method of sorting steel types. The pattern and colour of sparks produced when a piece of metal is touched against a grinding wheel can indicate the steel type. This method requires a good deal of experience for it to be a reliable sorting method and is not usually suitable for on-site use.
The assessment of mechanical properties usually involves specific testing techniques.
Relative hardness levels of materials can sometimes be used in sorting by checking the tendency of a surface to become scratched. There is unlikely to be a noticeable difference in the scratch resistance of different steel types, unless they have been heat treated to give high tensile and hardness levels. Although this could separate the softer ferritic, austenitic andduplex stainless steel from hardened martensitic stainless steels, it alone, would not distinguish between hardened martensitic stainless and non-stainless steel. The hardness of a piece of softened martensitic stainless steel would similar in a scratch test, to that of the other stainless steel type.
Effect of heating on properties
If there is a heat source capable of heating the steel to around 1000 degrees C (a light orange glow in subdued light conditions) then some complementary sorting tests can be done if there is a small piece of metal available for testing .
Heating to this light orange colour and quick cooling, preferably in water, can produce different properties, depending on the steel type. (Aluminium melts at 660 deg C so heating in this way would easily distinguish a sample from steels or nickel alloys)
An austenitic stainless steel that may have had some cold work before heating (some hardness and magnetic attraction) should show much less magnetic attraction after this heating and cooling cycle and be uniformly soft. Ferritic and duplex stainless steel will also be softened by this heating cycle, but no differences in magnetic attraction will be evident.
In contrast, if the steel is harder following this heating and cooling cycle (more scratch resistant) then indicates that the sample is probably a martensitic type. This does not in itself confirm that the steel is a martensitic stainless, as carbon and low alloy steel will also respond in this way. No differences in magnetic attraction will be evident on a martensitic steel, in the same way as ferritic and duplex steels.
Chemical (corrosion) Methods
It is important that the surfaces of metals being chemically tested are scale-free, coating-free, free of grease and any iron contamination and clean. Otherwise the test solution cannot interact properly with the metal surface.
Ideally the surface should be lightly abraded. ‘Wet and dry’ aluminium oxide based paper is suitable for this.
To make sure the surface is clean and grease free, simple washing in soapy water and rinsing in clean water, followed by drying with a clean paper tissue should be satisfactory. Alcohol based solvents can also be used for final degreasing.
A large drop of tap water left on a steel surface and left overnight will normally produce a rust stain on a carbon or low alloy steel, but not on a stainless steel. This will not however distinguish between different stainless steel families or grades.
Copper sulphate solution test
A simple 5 percent copper sulphate solution, applied in the same way as the water drop test, should confirm the differences between non-stainless steel and stainless steel. A metallic copper coloured deposit should form easily on non-stainless steels, but the solution should remain free of copper colour if the sample is a stainless steel.
Nitric acid test
Nitric acid is a more hazardous chemical to store and handle than copper sulphate and so is not a simple testing choice for on-site use. However a dilute solution of nitric acid will readily attack non-stainless steel, leaving most stainless steel unaffected. Some attack can indicate that the sample could be a martensitic type stainless, but this may not be conclusive. Concentrated nitric acid can be used to distinguish some nickel alloys from stainless steel, by the appearance of greenish-blue or pale green colours.
Acidified copper sulphate and copper chloride tests
ASTM A380 outlines a more refined test solution than the simple copper sulphate test. A 250 ml batch of test solution is made using distilled water and 10ml sulphuric acid sp gr 1.84 4g copper sulphate
If a copper deposit forms slowly, but during a period of 6 minutes of swabbing the steel surface with the solution, then this can indicate that the steel is likely to be either a ferritic or martensitic stainless steel. If not then the sample is likely to be an austenitic type. (This may also be the case for most duplex steel as they have similar or better passivity than austenitic types).
An alternative is a copper chloride solution acidified with hydrochloric acid. 13ml concentrated hydrochloric acid 10g copper chloride 50 ml distilled water
If the steel surface becomes copper plated when a drop is left on for one minute, then the sample is likely to be either a ferritic or martensitic stainless steel type. If not then the sample is likely to be an austenitic or duplex type.
The sulphur test
This method can be used to differentiate between sulphurized machinablity enhanced grades such as 1.4305 (303) and non-sulphurized grades like 1.4301 (304). The test will not differentiate between stainless and non-stainless machinability enhanced grades.
It is based on the ‘sulphur-print’ steel defect test, where a piece of slow speed photographic paper is soaked for a few minutes in dilute (approx 5%) sulphuric acid and then placed in contact with a clean steel surface. Sulphide inclusions in the steel react with the acid to form hydrogen sulphide gas, which can be seen on the paper as dark brown spot. If tests are done on samples of both steel the darker paper shows that sample to be a sulphurized grade. The papers can be developed and fixed with photographic solutions to confirm the ‘sulphur image’ but on-site, in-situ testing with the sulphuric acid soaked paper alone should be reasonably conclusive.
Other acid solution tests
There are other phosphoric acid, sulphuric acid and hydrochloric acid based tests that with the appropriate facilities and operator skill level can be used to sort various grades of stainless steel. These methods however usually involve using concentrated acids, some at high temperature and so are not appropriate for on-site grade sorting.
Proprietary chemical testing kits
Perhaps the most common mix of steel grades that needs to be sorted is the non-molybdenum austenitic 1.4301 (304) types from the molybdenum-containing 1.4401 (316) types. These tests produce distinctive colour changes depending on the presence of molybdenum in the steel.
More expensive but comprehensive chemical test kits intended to identify a range of stainless steel grades can be obtained from the USA. One such kit is the Alloy Detector 410L from Systems Scientific Laboratories Inc
Metals such as titanium or nickel alloys are relatively rare, except in particular industries such as aerospace or very demanding (chemically aggressive) processing plant. In more general engineering or building and construction applications low alloy (carbon) steel and stainless steel are more widely used. In this stepwise procedure we have assumed that the metal is assumed to be a steel, but of unknown type, with no grade or standard markings. Often a combination of tests is needed to narrow the choices to a type of grade.
Stainless steel is abbreviated to stainless steel
Low alloy or carbon steel is abbreviated to carbon steel
|1. Initial appearance||Paint coating or oiled surfaces||stainless steel is rarely painted. carbon steel sections often supplied primed, sometimes oiled to prevent corrosion|
|Bare metal surface with heavy grey scale or a rust covering||Mill produced stainless steel usually supplied descaled Unless contaminated with carbon steel a stainless steel surface will not show rust stains If the surface has general light rusting it is likely to be a carbon steel|
|2. Water drop or copper sulphate solution tests||Copper colour quickly develops||Steel very likely to be a carbon steel|
|3. Magneticattraction||A hand magnet is either not attracted or only weakly attracted in certain areas||Softened or moderately cold worked austenitic SS. Next use moly. spot test to show if it is a 304 or 316 type. A sulphur test will show if the steel is a free machining type, such as 303.|
|4. Acidified copper sulphate OR copper chloride test||Copper deposits slowly ie within a few minutes||Likely to be either ferritic or martensitic stainless steel, otherwise assume it is austenitic or duplex|
|5. Sulphur test||Paper shows distinct brown marks||Steel is a sulphurized grade (could be either carbon steel or stainless steel however)|
|6. Moly. spot test||Darkening of yellow spot test||Molybenum containing SS (316,317,444,904L,6% Mo types, and mostduplex steel)|
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)