Stainless Steel Alloys, Compositions, Processes And Finishes

1. Stainless Steel
2. Terminology

  • Annealing
  • As-Welded Anneale
  • As-Welded, Austenite
  • Bending, Bright Annealing
  • Carbide
  • Cold Drawing
  • Cold Worked (cw) And Cold Worked Annealed (cwa)
  • Defrasing (deburring)
  • Flanges
  • Fittings
  • Finishes
  • Nominal Bore
  • Non-destructive Testing
  • Ovalty
  • Passivating
  • Pickling
  • Pipe
  • Schedule Numbe
  • Section Tubing
  • Sinking
  • Tube

1. Stainless Steel:

1. Stainless Steel:
All grades of stainless steel from which tube, pipe and associated fittings are usually produced can be readily Welded by the common electric processes and with standard equipment High strength fabrications, seamless joining and any type of structural installations will normally be specified as requiring welding.

Further information can be obtained from Australian Welding Research Association (118 Alfred St. Milsons Point NSW. Aust. Such as their Tech Note 16 "Welding Stainless Steels" and AS 1554.6 "Welding Stainless Steels For Structural Purposes".

Carbide precipitation (also referred to as sensitization) is a phenomenon, which can occur if an austenitic (300 series) stainless steel is held in the temperature range 450 to 850"C, or cools slowly through this range. The result can be gain boundary precipitation of chromium carbide particles, which can reduce corrosion resistance. This particularly affects the heat-affected zones of welds.

In practice because tube, pipe and their fittings are generally fairly light sections it is not common to have a problem. For heavier section products it may be prudent to use extra low carbon grades; these are available as 304L and 316L.

It should be noted that although the low carbon variants 304L and 316L prevent the problem of carbide precipitation, their use at operating temperatures above 500"C is not recommended due to poor high temperature strength.

The incidence of carbide precipitation with its attendant consequences should not alarm welders. It can minimize in a number of ways. The speed of the weld, the technique, the use of cooling devices and other factors are all involved in the solution to this problem.

Discoloration of welds and associated heat affected zones by high temperature oxidation may be a problem; in general this heat tint can be removed by cleaning with a pickling solution. In many instances a "pickling paste" such as "Metinox 71 E" is more convenient For treating vertical or other non horizontal tubes and pipes and this product is available as a proprietary mix.

It must be stressed that all solutions and pastes used for pickling stainless steel are strong acids. Many contain the very dangerous chemical hydrofluoric acid, and in all instances these products must be handled carefully, appropriate safety gear including gloves and glasses worn and all residues very thoroughly rinsed off to an appropriate waste water stream.

2. Terminology

The heating and cooling of steel for the purpose of removing stresses, increasing ductility or changing the physical and metallurgical properties. Austenitic stainless steels such as Types 304 and 316 are annealed by heating to 1010 -1120 C and rapidly cooling. It is generally accepted that cooling from quenching temperature to black heat should be achieved within 3 minutes, although low carbon "L" grades do not need to meet this requirement. Maximum softness is obtained by quenching from the top annealing temperature range. Annealing of stainless steel is also termed "solution treatment" .

As-Welded Annealed:

Welded tube, which has been bright, annealed as a final operation - see also "Annealing" and "Bright"

As the name implies, this is tubing direct from the mill. It has a higher yield point than annealed tube and is generally used for decorative applications or in mildly corrosive conditions. It is not suitable for applications requiring significant flaring, expanding or bending.

A non-magnetic solid solution of carbon in Gamma iron. It is very unstable below its critical temperature for plain carbon steels but is retained in stable form at room temperature and even down to cryogenic temperatures in the austenitic stainless steels such as Types 304 and 316.

Bending is a most widely employed forming method; welded fittings are used for many short, tight turns, but there are still a great many places where bending is both desirable and necessary. Many plants, such as petrochemical plants, central station boilers, and refineries, have kilometers of tubing and piping which wander in complex paths. By using bends, many of the costs and hazards of joints can be avoided.

Although bending appears to be simple, it must be noted that the metal at the outer radius stretches and thins, but the metal at the inner radius is compressed and tends to thicken. If bending is poorly done, the metal at the outer radius can pull apart in a tensile-type failure or become too thin to meet design criteria. Since many high-pressure units must meet safety codes, thinning must be considered. A tube may actually fail if it lacks the required ductility during bending. Wrinkling or flattening at the inner radius may occur if care is not taken.

Size is a major factor. Tubes which have large diameter to wall thickness ratios may not require much power to deform them, but they are the most difficult to bend. When the ratio is small (as in most pipe) y more power may be needed, but in this instance the work piece tends to support itself and it will not as readily buckle, wrinkle or assume an oval shape. Size is especially important when working with the austenitic grades. These work harden more than the other grades; therefore, even more power may be needed to form them.

The geometry of a bend is also important. The ratio of the centreline bend radius to the tube's outer diameter (R/D) and the ratio of the diameter to the wall thickness (D/T) should be calculated. The resulting numbers will indicate, in large measure, whether a tube can be bent with no inner or outer support, or whether and what type of mandrels and outer support may sometimes be needed. Bending is probably not practical with R/D. i.e. for instance a 125mm diameter tube should not be bent to radius of or less regardless the wall thickness.

Bright Annealing:
Continuous annealing in a controlled atmosphere so that no oxide or scale is formed on the surface of the metal.

A complex chemical compound of carbon with chromium, iron or other elements, which are sometimes, found in stainless and heat resistant steels. The quantity and form of carbide can determine the properties of the steel. Chromium carbides present at the grain boundaries of austenitic stainless steels cause poor resistance to corrosion. This often serious problem is usually due to improper annealing or welding.

Cold Drawing:
The drawing of tubing through a hardened steel or tungsten carbide die while at room temperature. Cold drawing can be done with or without a plug or mandrel on the inside. The purpose of cold drawing is to Reduce the O.D. or wall or both, produce a smooth surface finish and to break up the weld structure, which results in re-crystallization when annealed.

Cold Worked (cw) And Cold Worked Annealed (cwa):
Tube for applications requiring a smooth internal surface, high corrosion resistance and compliance with specifications such as ASTM A249. Tube is available in cold worked (CW) and cold worked annealed (CWA] conditions. In both conditions the weld area is subject to a mechanical Forging process for gain refinement. CWA tube is then bright annealed, an economical alternative to welded drawn tube.

Defrasing (deburring):
The operation of removing from the pipe ends the ragged edges formed during cutting. DRAWN WELDED Welded Tubing which is subsequently cold drawn through a die and over a plug. Cold drawing removes the internal weld bead and breaks up the grain structure in the weld area so that optimum corrosion resistance is developed in the weld area after proper heat treatment. Cold drawing is used to produce intermediate sizes or to give higher tensile strength if tubes are not required in the annealed condition.

These components are used to provide a bolted connection to a tube. Flanges are available in highly Finished Stainless Steel. Forged Stainless Steel (and in Satin Chrome Plated for decorative purposes or mechanical installations.) Flanges of a flat plate profile are known as "Table" flanges and are specified by AS2129

A wide variety of Fittings is available to join pipe or tube to enable complex layouts such as wreath bends, changes of direction at stairs, turn down railing terminations or turned in wall railing terminations. Fittings are available as butt weld on or flush fitting mechanical which will enable subsequent disassembly. Fittings are specified by the dimensions of the tube to which they will be joined. They are listed in the Product Index of our Catalogue.

The natural metallic lustre of stainless steel, which remains during its extremely prolonged lifespan, can be further enhanced by polishing and buffing. This applies particularly in applications for decorative purposes. Maintenance is minimal and in most applications the surface finish needs only an occasional soap and water wash to restore surface lustre. A number of Finishes are available, as follows: MILL FINISH and off the mill finish. The 28 strip surface is retained; roll forming marks and an indication of the weld will be visible. BRIGHT ANNEALED Produced by first heat treating cold rolled steel in a controlled atmosphere, followed by moderate cold roll polishing. A bright, highly reflective finish. BUFF POLISH (BP) the most common tube finish specified. It is also referred to as "Satin Finish " or # 4. The standard polished tube Finish. A pleasing bright finish with medium lustre. Traces of external weld line and roll arks can be visible. Polished to approximately 180 grit finish or finer. On request tube can be polished to other finishes, such as 60, 120, 240, 320, 400 or 600 grit. MIRROR FINISH # 8 Mirror Finish tube has a blemish-free outside surface, for architectural application. This product can be produced on request but it is advised that this is an expensive labor intensive service

Nominal Bore:
In British and American practice there is a series of pipe sizes originally used for external threading to take fittings but described in terms of the bore diameter associated with each outside diameter. Pipe of a stated Nominal Bore therefore has the appropriate OD size listed in either BS1387 or ASTM A31 2. The term "Nominal Bore" or "NB" is unofficial; the correct term is the NPS designator.
It has become common practice in Australia to "metricate" NPs designations, which are given in ASTM and ANSI standards only in inches. These conversions must be used with caution as they are not raised by purchasing specifications and also because these conversions are very "nominal". They are however widely accepted in practice. See the Tables Section under Stainless Steel for equivalents.

Nondestructive Testing:
This embraces all types of testing and Inspection methods that do not destroy the material under test. It includes methods such as magnetic particle, radiography, pressure testing, ultrasonic and eddy current testing. Eddy current testing can be carried out as an in-line process to 100% check integrity of pipe, if requested.

Out-of-roundness or difference between maximum and minimum dimensions of Outside Diameter obtained by careful micrometer measurement for high and low points at any one section around the pipe or tube. The ovality tolerance is considered to be a total spread inside which both the maximum OD dimension and minimum OD dimension must fall. The outside diameter range specified includes an allowance for ovality. Specifications for thin walled product include wider tolerances for ovality.

The treating of stainless steel by dipping in a dilute solution of nitric acid such as S - WELD to remove free iron from the surface and improve the corrosion resistance.

Pickling is the immersion of a material in an acid solution for the purpose of removing oxide scale after annealing or heat treatment. Mixtures of nitric and hydrofluoric acid in water are the most effective and most widely used solutions for the removal of scale from all the austenitic grades of Stainless Steel. After pickling, the pipe is thoroughly rinsed/washed to remove all traces of acid. Sulfuric acid can be used to soften very tenacious oxides; this is normally then followed by nitric/hydrofluoric acid treatment. "Pickling Paste" can be used to pickle irregular, even vertical surfaces or to spot pickle areas such as welds.

Pipe can be defined as generally heavy wall thickness product made to dimensional standards usually designated by the nominal pipe size. It has fixed wall thickness and outside diameter established as standard by the American National Standards Institute (ANSI), the wall thickness depends upon the working pressures to be maintained. These wall thickness are designated as Schedule numbers.

Schedule 5S and IOS pipe is normally used where comparatively low pressures ore encountered and where butt welding fittings and connections are to be made. Schedule 40S and 80S pipe is used for high pressures and when threading is required. The term "Nominal Bore" can be misleading as both pipe and tube are specified by outside diameter and wall thickness. Some pipe manufacturers actually designate their product by outside diameter and wall thickness in millimeters

Schedule Number:
A system of defining the nominal wall thickness of pipe (sometimes therefore referred to as "schedule pipe"). The actual thickness of a given schedule also depends upon the size (NPs/NB or outside diameter) as given in tables in this publication. It should be noted that schedules usually quoted are "S" (for stainless) suffix, e.g. 5S, IOS, 40S and 80S. These schedules are detailed in ANSI 836.19.

Section Tubing:
Non-circular cross section tubing such as square, rectangles, ovals, etc., are first made in the round form and roll shaped or drawn as a supplementary operation, often by extra rolls on the tube rolling mill.

The process of reducing the diameters of tubes by drawing through dies without any internal support in the tube. Except for extremes in wall thickness (i.e. very thin or very thick) the wall thickness remains unaltered by sinking.

Tube is generally a lighter wall thickness product than pipe. It is defined by its outside diameter and wall thickness, specified in millimeters (or inches). The range of applications is very wide, and the most common profile used in balustrades decorative applications. The wide range of applications is matched by a wide selection of specifications. Tube is manufactured by seamless, longitudinally welded and spiral welded processes, in round, square and rectangular shapes. Supplementary processing may include drawing, cold working of the weld or polishing, as required for the application.


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