Stainless Steel - Fabrication

Topics Covered

Introduction
Work Hardening
    Magnetic Permeability
    Drawing
    Work Hardening Rates
    Slow Forming Speeds
Machining
    Improved Machinability Grades
    Rules for Machining Stainless Steels
Welding
    Austenitic Stainless Steels
    Duplex Stainless Steels
    Martensitic Stainless Steels
    Ferritic Stainless Steels
    Welding Dissimilar Metals
Soft Soldering
    Recommended Procedure for Soldering
Brazing (Silver Soldering)
    Recommended Procedure for Brazing

Introduction

Stainless steels, particularly austenitic grades, are capable of being fabricated by any conventional fabrication methods. The commonly used austenitic grades can be roll formed, spun, deep drawn, hot and cold forged or bent and folded with a strong force, owing to the material’s high strength and work hardening rate.

Despite these properties, austenitic stainless steels have very high ductility, which enables them to be very heavily cold formed into deep drawn products. Some of the other metals are also capable of being cold formed without splitting.

Work Hardening

All metals can be work hardened upon cold working, based on the type of stainless steel grade. Austenitic stainless steels tend to work harden at a rapid rate, but the cold working rate of 400 series stainless steel is slightly higher than that of the plain carbon steels. The rapid cold working rate of austenitic steels makes them suitable for applications that involve high corrosion resistance and strength, such as spring manufacture in corrosive environments. The relationship between the extent of cold working and the resulting mechanical properties are represented in the chart of Figure 1.

Figure 1 - illustrates the relationship between the amount of c old work (expressed as "% reduction of area") and tensile strength.

It should be noted that austenitic stainless steels can be hardened only by work hardening. However, like low alloy and carbon steels, martensitic stainless steels can be hardened by thermal treatments such as quenching and tempering. Cold working can harden ferritic stainless steels, like austenitic grades. Their work hardening rates, however, are very low and, therefore, it is extremely difficult to achieve high strength.

Magnetic Permeability

The steel grades with the highest work hardening rate usually have the highest magnetic permeability for a given amount of cold work.

Drawing

The tensile properties of stainless steel grades, such as grade 301, 302 and 304, can be enhanced up to 2000 MPa in cold working treatments such as cold drawing. However, such high tensile strength values are limited to fine wire sizes and thin sections.

With the increase in size of a material, cold working necessary to achieve high tensile properties will not be suitable, owing to the rapid work hardening rate of large surface sections. For instance, 6 mm round, cold drawn grade 304 steel with 15% reduction in area will have an ultimate strength of about 800 MPa. By contrast a 60 mm round, cold drawn grade 304 steel with same reduction percentage will show the same ultimate strength in full section. However, the 60 mm round section will exhibit low tensile properties if it is machined from its centre, while the 6 mm round section will have about the same in full section. Moreover, the 6 mm round steel can be cold worked to achieve high tensile properties, whereas the 60 mm round section should be annealed for further cold working, owing to high hardness.

Work Hardening Rates

400 series alloys are magnetic at room temperatures, and they will work harden at rates similar to that of low carbon steels. Wire products of this series can be cold worked to achieve tensile properties as high as around 1000 MPa. However, bar products are often cold worked at a rate higher than 850 MPa. Ferritic steel grades cannot be heat-treated, but martensitic steel grades are heat-treated using hardening and tempering, to achieve maximum corrosion resistance and mechanical properties. The work hardening rates tend to decrease with an increase in temperature. This change in rate can be observed at low temperatures of 80°C.

Slow Forming Speeds

Unlike carbon steels with the same formability at any operating condition, stainless steels experience severe deformation at slow forming speeds during cold working.

Machining

In general, austenitic stainless steel grades are difficult to machine. For this reason, fabricators developed a free-machining grade 303 steel. Free-machining versions are also available for martensitic and ferritic grades. Grades 416 and 430F exhibit improved machinability, due to the presence of manganese sulphide that serves as chip breakers.

The free-machining grades offer lesser corrosion resistance when compared to that of non-free-machining grades, due to the presence of non-metallic inclusions. These grades should not be used in aggressive conditions, such as marine environments, as they are subjected to pitting corrosion attacks. Those grades with high sulfur levels have reduced ductility and, hence, they cannot be bent, welded or cold forged.

Improved Machinability Grades

A number of manufacturers have produced “improved machinability” versions of austenitic stainless steel grades. Such as grade 304 and 316, by using steel melting techniques. These techniques seem to have sufficient chip-breaking effect for significantly improving the machinability. However, they retain the mechanical properties, such as corrosion resistance, formability and weldability, of their standard grades. These materials are commercially available in trade names of "Ugima". The machining speed of "Ugima" is nearly 20% higher than that of other standard equivalent grades.

"Ugima 303" is commonly referred as a "super-machinable" grade. The corrosion resistance, formability and weldability of “Ugima 303” are compromised to achieve maximum machinability. The graph in figure 2 represents the relative machinabilities of various stainless steels denoted in comparison of achievable cutting speeds.

Figure 2 - illustrates relative machinability ranges of stainless steels

Rules for Machining Stainless Steels

Given below are some general rules of machining that are applicable to most of the stainless steels:

• Use of proper lubricants and coolants are necessary. A large amount of heat generated during machining austenitic alloys will be concentrated at the cutting edges of the tools due to low austenitic steels.
• Constant feeds are essential to ensure proper placement of work.
• Large tools can be used to aid heat dissipation.
• There should be a large number of clearances to prevent the tool from hindering the work.
• Although light cuts need to be taken, the depth of each cut should be sufficient enough to prevent the tool from promoting work hardening.
• The cutting edge of tools needs to be kept sharp, as dull tools promote glazing and work hardening of the metal surface. Sharpening should be carried out by machine grinding immediately after the quality of the cut deteriorates.
• The machine tool needs to be sturdy and vibration-free, and it must have sufficient power.

Welding

Stainless steels have different weldabilities, and almost all stainless steels can be welded. Austenitic grades are the most readily welded metals. The weldability of stainless steels depends on the family that they belong to. Australian Standard AS 1554.6 provides a number of pre-qualified conditions for welding metals and covers structural welding of stainless steels. Pre-qualified welding consumables for welding different metals, or the same metal, can be referred to from Table 4.5.1 of AS 1554.6. This standard also covers the specification of welding procedures that suits every application.

Austenitic Stainless Steels

The austenitic steel grades are readily welded through all conventional electric welding procedures, using a wide range of welding consumables and standard equipment. The use of stabilized grades, or grades with low carbon content, for welding heavy section products resolve the problem of inter-granular corrosion and sensitization. Thin materials can be quickly welded as sensitization depends on time or temperature. Care should be taken that, if fabrication has become sensitized during welding, corrosion resistance of the material can be restored by a full solution treatment.

The free-machining grade 303 is subjected to hot cracking and, hence, it is not preferred for welding applications. The “Ugima” improved machinability grades, “Ugima 304” and “Ugima 316”, offer reasonable machinability and excellent weldability.

Duplex Stainless Steels

Duplex stainless steels have good weldability, although not as good as that of austenitic grades. All standard welding methods can be used along with a wide variety of consumables. One of the advantages of duplex stainless steels is that they have low thermal expansion coefficient when compared to that of austenitic stainless steel.

Martensitic Stainless Steels

Martensitic stainless steel grades (except high sulphur free-machining grade 416) can be welded often with austenitic filler rods for improving the ductility of the steels. However, care must be taken as they form brittle and hard zones adjacent to the weld. Cracking can also occur in these zones, and hence caution needs to be exercised with pre-heating and post-welding treatments.

Ferritic Stainless Steels

The ferritic grades do not possess good welding properties due to lack of ductility, sensitization and excessive grain growth. Post-weld heat treatment can be used to solve some of these problems. Filler metal, which helps in enhancing weld toughness, can be of austenitic grade or of similar composition. The excessive grain growth problem is extremely difficult to overcome, and, hence, most of the grades are welded only in thin gauges. Stabilized ferritic grades exhibit better weldability when compared to that of unstabilized grades.

Grade 3CR12 is a proprietary ferritic grade with very low carbon content. It can be readily welded even in a heavy section plate. As for other ferritic grades, it is normal to use austenitic stainless steel fillers.

Welding Dissimilar Metals

Welding of different metals, such as grades 304 and 430, can be performed with some precautions. Over-alloyed austenitic welding rods, such as grade 309, are recommended to reduce dilution effects on stainless steels. The Schaeffler diagram shows the composition of the weld deposit generated from dissimilar grade welding. AS 1554.6 provides a table that specifies pre-qualified consumables for each combination of dissimilar metal welds.

Soft Soldering

A lead-tin soft solder can be used for soldering all grades of stainless steel. Leaded solders should be avoided if the product is being soldered for applications such as transport, serving or food processing. Soldered joints are relatively weak over the strength of steel, and, hence, this method should not be used where the mechanical strength is dependent upon the soldered joint. Strength can be improved if the edges are riveted, spot welded or lock-seamed.

Recommended Procedure for Soldering

The recommended procedure for soldering is as follows:

• The steel surfaces must be clean and free of oxidation.
• A rough surface enhances solder adherence, and, hence, roughening with coarse abrasive paper, file or grinding wheel is preferred.
• Phosphoric acid-based flux is recommended. Hydrochloric acid based fluxes are not recommended as they need to be neutralized after soldering.
• Flux should be done using a brush only to the area being soldered.
• A large, hot iron can be used. The temperature for soldering stainless steel can be maintained as that of carbon steel, but a longer time will be required, owing to the low thermal conductivity of stainless steel.
• Any type of solder can be used, but at least 50% tin is recommended. Solder with 30-40% lead and 60-70% tin has greater strength and better colour match.

Brazing (Silver Soldering)

Brazing is used when welding is impossible and a strong joint is needed. It is useful for joining nickel, bronze, stainless steel, copper and other non-ferrous metals. The corrosion resistance of a joint produced by brazing will be slightly lower than that of the stainless steel, but brazed joints meet the requirements under atmospheric and mildly corrosive operating conditions.

Recommended Procedure for Brazing

The following is the recommended procedure for brazing:

• Use silver brazing alloys with melting points from 590-870°C.
• Remove dirt and oxides from the steel surfaces and apply flux immediately.
• A slightly reduced flame should be used to uniformly heat the joint.
• For high production work, use controlled atmosphere furnaces or induction heating.
• Remove all remaining flux using hot water, or high-pressure steam, after brazing.
• When brazing grade 430 alloys, use silver solder containing 3% nickel. In combination with austenitic grades, this alloy helps to reduce crevice corrosion.

The following table outlines various recommendations for welding stainless steels.

Table 1 - Recommendations for welding of stainless steels

Notes

1. Unnecessary when the steel is above 15°C.
2. Where corrosion is a factor, 309S and 310S (0.08% carbon maximum) are used, with a post-weld heat treatment of cooling rapidly from 1120-1180°C.
3. Pre-heat at 200-320°C; a light gauge sheet is frequently welded without pre-heat.
4. May be welded with 308L, 309 or 310 electrodes without pre-heat if the steel is above 15°C.
5. May be welded with 309, 309L, 309Mo, 309MoL, 316L or 308L.
6. If temperature below 10°C a 50°C pre-heat is recommended.