Glossary of Terms

Anneal

A heat treatment to fully soften the material and/or to dissolveand take back into equilibrium solution any intermetallic compounds (eg carbides) which may have formed within the crystal structure. For most metals and alloys this involves heating to a specified high temperature and subsequently cooling at a slow rate. However, for austenitic stainless steels the subsequent cooling must be rapidly effected.

Bright annealing

Using a furnace with an inert atmosphere no scale is formed and the resulting surface has a bright finish.

Carbide

A compound formed when an element combines with carbon. The carbides of metals are usually intensely hard.

Carbide precipitation

chemical reaction whereby the intermetallic carbides are formed within the crystal structure. They are hard particles which impart hardness and abrasion resisting properties. However, in stainless steels heating to within a high temperature range (±450°-850°C) causes the formation chromium carbide. This takes place preferentially at the grain boundaries. A small amount of carbon locks up a large amount of chromium. The material is thus “sensitised”. The chromium depleted grain boundaries are therefore prone to suffer preferential and accelerated corrosive attack along the grain boundaries (intergranular corrosion).

Cold drawing

The process is effected without the metal being first heated to high temperatures. Examples of the process are the drawing of a tube through a die and over a mandrel to reduce its OD and/or wall thickness, or drawing wire through successive dies in series to reduce its diameter.

Creep

The slow and continuous deformation of a metal at high temperatures. The deformation (strain) is dependent on the stress, the temperature and time. At the high temperatures creep can occur at very low levels of stress, less than 10MPa.

Ductility

The property of a metal to deform in a plastic manner (ie undergo permanent strain) without fracturing. Elongation and reduction of area (RA) are reported properties which give an indication of the ductility.

Electropolishing

A process whereby the metal surface is actually corroded away under very carefully controlled electrolytic conditions. A smoothing and levelling of the surface takes place. Burrs are also removed from any rough edges. The surface of electropolished stainless steel has a high degree of reflectivity, and the passive film produced on the surface is superior (more corrosion resistant) owing to the formation of oxygen gas at the surface during the process.

Extrusion

Aprocess which confers a given shape on the cross section of a length of metal by pushing the metal, in the solid but plastic state, through a die.

Fatigue

If metals are subjected to repeated fluctuating (reversing) loads at stresses below the tensile strength, a fatigue crack can initiate in the material which, with increasing number of loading cycles, propagates through the material until final failure by fracture of the metal remaining occurs. Fatigue is an interrelationship between stress and the number of load cycles, the lower the stress the greater the number of cycles that can be tolerated (in some metals a stress below a certain limiting stress value will never induce fatigue). Failure by fatigue is very sensitive to any surface defects or imperfections which radically lower the fatigue resistance.

Free machining grades

Brought about the the addition of sulphur or selenium, increases cutting speeds by approximately 75% on stainless steel. Sulphur is preferred for heavy machining because of the large and fairly continuous inclusions. Selenium is better for light work where a good finish is required.

Hardenable

This means that a material can be hardened by heat treatment which involves heating the material to a specified high temperature and subsequently cooling it (quenching) at a rapid rate. Quenching must be followed by tempering in order to develop the correct required combination of strength, hardness, ductility and toughness.

Heat affected zone (HAZ)

This is within the parent metal adjacent to the weld metal (deposit). It is heated through a range of temperatures up to the melting point which occurs at the junction with the weld metal. The actual temperature attained at any point within the HAZ is an interrelated factor of distance from the weld and the amount of heat input. The high temperatures induced cause changes to and within the crystal structure of the parent metal, which in turn affect the mechanical, physical and corrosion properties within the HAZ.

Heat treatment

Any high temperature treatment of a metal or alloy in order to modify (improve) the mechanical properties, (and sometimes the physical properties).

Mechanical properties

A measure of the metal’s response to an applied force or load (ie stress). The commonly reported mechanical properties include yield strength, tensile strength, elongation, reduction of area (RA), hardness, toughness (Charpy V) and fatigue.

Passive film

Chromium contents in excess of ±11% Cr in stainless steels result in the formation of a chromium oxide passive film on the surface, provided there is a sufficient availability of oxygen for its formation. This passive film is extremely thin, continuous, tenacious, stable and self-repairable. It renders the surface inert to many chemical reactions and therefore passive. This is stainless steel’s natural built-in corrosion resistance.

Passivation

The treatment of the surface of stainless steels with dilute solutions (or pastes) of nitric acid HNO3. This, being an oxidising acid, promotes the formation and improves the integrity of the passive film on any freshly created surface (eg through grinding, machining or mechanical damage). The acid treatment also has the secondary beneficial effect of dissolving any free iron or steel contamination which may have been picked up during handling, forming or fabrication operations, and if this were not removed would impair the corrosion resistance. Nitric acid is the only acid which should be used to effect passivation of stainless steels.

Physical properties

Defined as the properties other than mechanical that pertain to the physics of a material, eg density, electrical conductivity, heat conductivity and thermal expansion.

Pickling

The removal of the oxide film from the surface of a metal by chemical means. An exposure to high temperature (eg during welding or heat treatment) will scale the surface. In the case of stainless steel such high temperature scale has inferior corrosion resistance and must be removed. Pickling, using formulations of hydrofluoric (HF) and nitric HNO3 acids, removes the scale and restores the corrosion resistance. For applications in aggressive environments it is advisable to develop full corrosion resistance by a passivation treatment subsequent to the pickling operation.

Solution treatment

A heat treatment which effects the solution of intermetallic compounds or precipitates (eg carbides) at high temperatures. Subsequent cooling must be fast enough to prevent their reformation during the cooling cycle.

Stabilised

This refers to the alloying of titanium (Ti) or niobium (Nb) to the austenitic grades. These elements form stable carbides, thereby locking up the carbon and preventing the formation of chromium carbides. Prevents sensitisation and intergranular corrosion (weld decay) in the region next to the weld in welded components of thicker material (> ±2.5mm).

Tempering

Quenched materials are hard and strong, but extremely brittle and of low ductility. Tempering should immediately follow quenching, and be effected at a temperature necessary to increase the toughness and ductility. This will usually incur a loss of hardness and strength, more so if higher tempering temperatures are used. The tempering is chosen to bring about the correct desired combination of properties. The maximum tempering temperature is below that at which change of crystal structure will be induced.

Tempers

Used to define the levels to which austenitic stainless steels are strengthened by cold rolling or cold drawing, without any subsequent annealing operation. Cold rolled sheet, coil and strip are produced to 1/4, 1/2, 3/4 and full hard tempers; wire is produced to annealed, soft, intermediate and spring tempers.

Tensile strength

The value of the maximum stress in tension that a material will withstand.

Weldability

This is not an exactly quantifiable or precise property, but rather implies the ability of the material to be joined by standard welding processes so that the resultant mechanical, physical and chemical properties of the weld zone (ie both the weld metal and the HAZ) are at least equivalent to those of the parent metal.

Work hardening

Most metals and alloys will exhibit a slight degree of increase in both strength and hardness if subjected to cold work (eg cold rolling, cold drawing, cold bending etc). The austenitic stainless steels show a marked response to work hardening and are therefore termed ‘hardenable by cold work’ or ‘work hardenable’ materials.

Yield strength

At a stress below the tensile strength the material reaches a point at which permanent strain (deformation) occurs. In some steels there is a marked increase of strain for no increase in stress ie yield point. If this does not occur (as in austenitic grades of stainless steel) a stress value for a specified amount of strain (usually 0.2%) is taken. The stress at the yield point or at 0.2% strain is reported as the yield strength.