Background

One of the major advantages of the stainless steels, and the austenitic grades in particular, is their ability to be fabricated by all the standard fabrication techniques, in some cases more severely than the more well-known carbon steels. The common austenitic grades can be folded, bent, cold and hot forged, deep drawn, spun and roll formed. Because of the materials' high strength and very high work hardening rate all of these operations require more force than for carbon steels, so a heavier machine may be needed, and more allowance may need to be made for spring-back.

Austenitic stainless steels also have very high ductilities, so are in fact capable of being very heavily cold formed, despite their high strengths and high work hardening rates, into items such as deep drawn laundry troughs. Few other metals are capable of achieving this degree of deformation without splitting.
Work Hardening

All metals work harden when cold worked and the extent of work hardening depends upon the grade selected. Austenitic stainless steels work harden very rapidly, but the 400 Series' cold working rate is only a little higher than that of the plain carbon steels. The rapid cold working characteristics of austenitic stainless alloys makes them particularly useful where the combination of high strength and corrosion resistance are required, such as for manufacture of springs for corrosive environments. The relationship between the amount of cold work (expressed as "% reduction of area") and the resulting mechanical properties is shown in the chart in Figure 1.

Figure 1. Cold work (expressed as "% reduction of area") versus tensile strength.

It is important to realise that work hardening is the only way in which austenitic stainless steels can be hardened. By contrast the martensitic stainless steels (410, 416, 420 and 431) can be hardened by a quench-and-temper thermal treatment in the same way as carbon and low alloy steels. Ferritic stainless steels (such as Grade 430) are similar to austenitic grades in that they can only be hardened by cold working, but their work hardening rates are low, and a substantial lift in strength cannot be achieved.
Magnetic Permeability

In general, the alloys showing the greatest work hardening rate will also have the highest magnetic permeability for a given amount of cold work.
Drawing

In cold working such as cold drawing, tensile properties over 2000 MPa may be obtained with Grades 301, 302 and 304. However, these very high tensile properties are limited to thin sections and to fine wire sizes.

As the size increases, the amount of cold work necessary to produce the higher tensile properties cannot be practically applied. This is due to the fact that the surface of larger sections rapidly work hardens to the extent that further work is not practical, while the centre of the section is still comparatively soft. To illustrate this point, Grade 304 6mm round, cold drawn with 15 per cent reduction in area will show an ultimate strength of about 800 MPa. A 60mm round, drawn with the same reduction will have about the same tensile in full section. However, if sections are machined from the centre of each bar, the 60mm round will show much lower tensile properties, whereas the 6mm round will test about the same as in full section. Also the 6mm round can be cold worked to much higher tensile properties, whereas the 60mm round must be annealed for further cold work, because of excessive skin hardness.
Work Hardening Rates

400 Series alloys work harden at rates similar to low carbon steel and are magnetic in all conditions at room temperatures. Wire sizes may be cold worked to tensile properties as high as approximately 1000 MPa. However, bar sizes are seldom cold worked higher than 850 MPa. Although the ferritic grades (e.g. 430, 409 and 3CR12) cannot be heat treated, the martensitic grades (e.g. 410, 416 and 431) are usually heat treated by hardening and tempering to develop mechanical properties and maximum corrosion resistance. Cold working, therefore, is more of a sizing operation than a method of producing mechanical properties with these grades.

The rate of work hardening, while relatively consistent for a single analysis, will show a marked decrease in the rate as the temperature increases. This difference is noticeable at temperatures as low as 80°C. Advantage is taken of this in some deep drawing applications as well as in "warm" heading of some difficult fasteners.
Slow Forming Speeds

Another feature of cold forming of stainless steels is that more severe deformation is possible at slower forming speeds - this is quite different from carbon steels which have formabilities virtually the same no matter what the forming rate. So the advice given to those attempting difficult cold heading (or other high speed forming operations) is to slow down; stainless steel is almost always headed slower than is carbon steel.
Machining

Austenitic stainless steels are generally regarded as being difficult to machine, and this has led to the development of the free-machining Grade 303. There are also free-machining versions of the standard ferritic (Grade 430) and martensitic (Grade 410) grades - Grades 430F and 416 - these grades have improved machinability because of the inclusions of Manganese Sulphide (formed from the Sulphur added to the steel) which act as chip breakers.

The free-machining grades have significantly lower corrosion resistances than their non-free machining equivalents because of the presence of these non-metallic inclusions; these grades are particularly prone to pitting corrosion attack and must not be used in aggressive environments such as for marine exposure. The free-machining grades containing high sulphur levels also have reduced ductility, so cannot be bent around a tight radius nor cold forged. Because of the sulphur additions these grades are very difficult to weld, so again would not be chosen for welded fabrication.
Improved Machinability Grades

Recently a number of manufacturers have offered "Improved Machinability" versions of the standard austenitic Grades 304 and 316. These steels are produced by proprietary steel melting techniques which provide enough of a chip-breaking effect to significantly improve the machinability, but they still remain within the standard grade composition specifications and still retain mechanical properties, weldability, formability and corrosion resistance of their standard grade equivalents. These materials are marketed under trade names such as "Ugima". For "Ugima" the improvement in achievable machining speed is about 20% over the equivalent standard grades; in addition it is commonly experienced that greatly enhanced tool life is obtained, which considerably reduces the cost of machining. In many instances this is of greater benefit than is the improvement in cutting speed.

A "Ugima 303" is available as a "super-machinable" grade; like other 303 stainless steels weldability, formability and corrosion resistance are compromised in order to achieve maximum machinability. Relative machinabilities of various stainless steels, expressed as comparison of achievable cutting speeds, are shown in the graph of figure 2.

Figure 2. Relative machinability ranges (in orange) of stainless steels

Rules for Machining Stainless Steels

Some general rules apply to most machining of stainless steels:

· 1. The machine tool must be sturdy, have sufficient power and be free from vibration.

· 2. The cutting edge must be kept sharp at all times. Dull tools cause glazing and work hardening of the surface. Sharpening must be carried out as soon as the quality of the cut deteriorates. Sharpening should be by machine grinding using suitable fixtures, as free-hand sharpening does not give consistent and long-lasting edges. Grinding wheels must be dressed and not contaminated.

· 3. Light cuts should be taken, but the depth of the cut should be substantial enough to prevent the tool from riding the surface of the work - a condition which promotes work hardening.

· 4. All clearances should be sufficient to prevent the tool from rubbing on the work.

· 5. Tools should be as large as possible to help to dissipate the heat.

· 6. Chip breakers or chip curlers prevent the chips from being directed into the work.

· 7. Constant feeds are most important to prevent the tool from riding on the work.

· 8. Proper coolants and lubricants are essential. The low thermal conductivity of austenitic stainless alloys causes a large percentage of the generated heat to be concentrated at the cutting edges of the tools. Fluids must be used in sufficient quantities and directed so as to flood both the tool and the work.
Welding

The weldabilities of the various grades of stainless steels vary considerably. Nearly all can be welded, and the austenitic grades are some of the most readily welded of all metals. In general the stainless steels have weldabilities which depend upon the family to which they belong. Australian Standard AS 1554.6 covers structural welding of stainless steels, and gives a number of pre-qualified conditions for welding. Pre-qualified welding consumables for welding of same-metal and mixed-metal welding are given in Table 4.5.1 of AS 1554.6. This excellent standard also enables specification of welding procedures appropriate to each particular application.
Austenitic Stainless Steels

The austenitic grades are all very readily welded (with the exception of the free-machining grade 303 noted elsewhere). All the usual electric welding processes can be used. A full range of welding consumables is readily available and standard equipment can be used.

The use of low carbon content grades (304L and 316L) or stabilised grades (321 or 347) needs to be considered for heavy section product which is to be welded. This overcomes the problem of "sensitisation" and intergranular corrosion. As the sensitisation problem is time/temperature dependent, so thin materials, which are welded quickly, are not usually a problem. It should be noted that if a fabrication has become sensitised during welding the effect can be reversed and the material restored to full corrosion resistance by a full solution treatment.

The free-machining grade Grade 303 is not recommended for welded applications as it is subject to hot cracking; the Ugima improved machinability grades, Ugima 304 and Ugima 316, offer a much better combination of reasonable machinability with excellent weldability.
Duplex Stainless Steels

Duplex stainless steels also have good weldability, albeit not quite as good as that of the austenitics. Again all the usual processes can be used, and a range of consumables is available. For the most common duplex grade 2205 the standard consumable is a 2209 - the higher nickel content ensures the correct 50/50 ferrite/austenite structure in the weld deposit, thus maintaining strength, ductility and corrosion resistance. One of the advantages of duplex stainless steels over austenitics is their comparatively low coefficient of thermal expansion. This closely matches that of carbon steels, as shown in the table in the section of this handbook on high temperature properties of stainless steels.
Martensitic Stainless Steels

Martensitic stainless steels can be welded (again with the high sulphur free machining grade 416 being not recommended) but caution needs to be exercised as they will produce a very hard and brittle zone adjacent to the weld. Cracking in this zone can occur unless much care is taken with pre-heating and with post weld heat treatment. These steels are often welded with austenitic filler rods to increase the ductility of the deposit.
Ferritic Stainless Steels

The ferritic grades again do not possess good welding properties. The three major problems encountered are excessive grain growth, sensitisation and lack of ductility. Some of these problems can be minimised by post-weld heat treatment. Filler metal can be of either a similar composition or alternatively an austenitic grade (e.g. Grades 308L, 309, 316L or 310) which is helpful in improving weld toughness. The excessive grain growth problem is difficult to overcome, so most grades are only welded in thin gauges. Stabilised ferritic grades include 409 and 430Ti. These possess considerably better weldability compared to the unstabilised alternatives such as 430. These grades can be welded, but certainly not as readily as the austenitic grades

3CR12 is a proprietary ferritic (actually dual-phase ferritic plus some martensite) grade which has a very low carbon content and has the remaining composition and the mill processing route balanced to enable welding. 3CR12 is quite readily welded even in heavy section plate. As for other ferritic grades it is normal to use austenitic stainless steel fillers.
Welding Dissimilar Metals

Welding together of different metals, such as of Grade 304 to Grade 430 or a stainless steel to a mild steel, can be carried out, although some extra precautions need to be taken. It is usually recommended that over-alloyed austenitic welding rods, such as Grade 309, be used to minimise dilution effects on the stainless steel component. The composition of the weld deposit resulting from dissimilar grade welding is shown in the Schaeffler diagram or its successors by De Long and more recently the WRC. AS 1554.6 contains a table giving the pre-qualified consumables for each combination of dissimilar metal welds
Soft Soldering

All grades of stainless steel can be soldered with lead-tin soft solder. Leaded solders should not be used when the product being soldered is used for food processing, serving or transport. Soldered joints are relatively weak compared to the strength of the steel, so this method should not be used where the mechanical strength is dependent upon the soldered joint. Strength can be added if the edges are first lock-seamed, spot welded or riveted. In general welding is always preferable to soldering.
Recommended Procedure for Soldering

Recommended procedure for soldering:

· 1. The steel surfaces must be clean and free of oxidation.

· 2. A rough surface improves adherence of the solder, so roughening with grinding wheel, file or coarse abrasive paper is recommended.

· 3. Use a phosphoric acid based flux. Hydrochloric acid based fluxes require neutralising after soldering as any remnant traces will be highly corrosive to the steel. Hydrochloric acid based fluxes are not recommended for soldering of stainless steels.

· 4. Flux should be applied with a brush, to only the area being soldered.

· 5. A large, hot iron is recommended. Use the same temperature as for carbon steel, but a longer time will be required because of stainless steel's low thermal conductivity.

· 6. Any type of solder can be used, but at least 50% tin is recommended. Solder with 60-70% tin and 30-40% lead has a better colour match and greater strength.
Brazing (Silver Soldering)

When welding is impractical and a stronger joint than soft soldering is required, brazing may be employed. This method is particularly useful for joining copper, bronze, nickel and other non-ferrous metals to stainless steel. The corrosion resistance of the joint will be somewhat lower than that of the stainless steel, but in normal atmospheric and mildly corrosive conditions brazed joints are satisfactory. Because most brazing operations involve temperatures at which carbide precipitation (sensitisation) can occur in the austenitic grades, low carbon or stabilised grades (304L, 316L or 321) should be used. Ferritic grades such as 430 and 3CR12 can be quenched from the brazing temperatures, but hardenable martensitic grades (410, 420, 431) should not be heated above 760°C when brazing. The free machining grades 303, 416 and 430F should generally not be used as a dark scum forms on the surface when fluxing and heating, which adversely affects the appearance of the steel.
Recommended Procedure for Brazing

Recommended procedure for brazing:

· 1. Use silver brazing alloys with melting points from 590-870°C. Select the alloy for best colour match.

· 2. Remove dirt and oxides from the steel surfaces and apply flux immediately.

· 3. A slightly reducing flame should be played across the joint to heat uniformly.

· 4. For high production work use induction heating or controlled atmosphere furnaces (argon, helium or dissociated ammonia with dew point of about -50°C).

· 5. After brazing remove all remaining flux with high pressure steam or hot water.

· 6. When brazing grade 430 use a silver solder with 3% nickel. This alloy also helps to minimise crevice corrosion when used with austenitic grades.