Glossary of Metallurgical Terms

BP Exploration

Appendix 2 Metallurgical Glossary Of Terms

BP Exploration

Appendix 2 Metallurgical Glossary Of Terms

APPENDIX 2

METALLURGICAL GLOSSARY OF TERMS

Appendix 2 METALLURGICAL GLOSSARY OF TERMS

AISI Steel Grades. The AISI system of classifying grades of carbon and low-alloy steel uses a four-digit number to denote the steel composition. Examples likely to be encountered in the oil production industry are:

The 10XX series

These are plain carbon steels with a maximum Manganese content of 1.0%. The XX refers to the nominal carbon content. For example, 1020 would have a nominal carbon content of 0.2%.

The 41XX series

These are lowalloy steels containing chromium and molybdenum. The XX refers to the nominal carbon content. For example, 4137H has a nominal composition of 0.95%Cr, 0.20%Mo and 0.37%C. The H denotes a special hardenability requirement.

The 43XX series

These are Ni/Cr/Mo containing low-alloy steels. Once again, the XX denotes the nominal carbon content. For example, 4340 has a nominal composition of 1.82%Ni, 0.80%Cr, 0.25%Mo and 0.40%C.

AnnealingThis is a heat treatment process to which steels are often subjected. It involves heating the steel to a predefined level, holding it at this temperature for a specified time, then slow cooling (usually by simply turning the furnace off, known as a furnace cool). The purpose of annealing is primarily to remove any cold work/residual stress within the steel. In addition, there is normally some grain refinement (reduction in grain size). Annealing results in an increase in the ductility of the steel.

API Materials GradesThe API Standard for casing and tubing (API 5CT) contains materials classification grades. These consist of a letter followed by a two or three digit number, e.g. P110. The number designates the specified minimum yield strength (YS) in ksi (thousands of pounds per square inch).

AusteniteHigh temperature phase of iron which has a facecentred cubic crystallographic structure. In steels, the solute is generally carbon. Austenite is not generally stable at room temperature, in plain carbon steels, it is not stable below 723C. However, it can be stabilised by alloying, e.g. austenitic stainless steel, in which nickel is the stabilising alloying element.

Austenitic Stainless SteelA stainless steel in which austenite is the stable phase at room temperature. These normally contain chromium in the range 16-26% and nickel in the range 6-20%. These alloys can contain some ferrite (up to 5%), which can adversely affect their corrosion resistance and weldability. These steels can not be hardened by quenching, but can only be strengthened by cold work.

Carbon SteelSteel that owes its properties chiefly to the carbon content of the material rather than the presence of other alloying elements which are seldom present in appreciable amounts.

Charpy TestAn impact test in which a notched bar sample, fixed at both ends, is struck by a falling pendulum. The energy absorbed, as determined by the subsequent rise of the pendulum, is a measure of the impact strength or notch toughness. this value is known as the Charpy Impact value and is normally quoted in either Joules or footpounds. The test temperature and specimen orientation are also important parameters and these are also quoted. See also Drilling Operations Bulletin No 47.

Cold WorkingThe plastic deformation of a metal at a temperature low enough to cause permanent strain-hardening. The hardness and tensile strength are progressively increased with the amount of cold work, but the ductility and impact strength (toughness) are reduced. Cold working is the technique often used to obtain the necessary strength in the corrosionresistant alloys, e.g. duplex stainless steel and the more highly alloyed austenitic stainless steels such as Sanicro 28.

Duplex Stainless SteelStainless steels in which there is a two-phase structure of ferrite and austenite. These are normally present in balanced or nearbalanced quantities (50%/50%). Typically these steels contain 22 to 25% chromium and 5 to 7% nickel e.g. Alloy 2205, Alloy 2507.

ElongationIn tensile testing the extension of the testpiece when stressed to fracture, usually expressed as a percentage of a specified gauge length (e.g. x% on a gauge length of 2 inches). This is a measure of the ductility of the material.

ERW (Electric Resistance Welded) Tubulars

Tubulars made by forming a strip of metal into a tube and then joining the longitudinal edges by electricresistance welding (ERW). In ERW the mating longitudinal edges are heated to high temperatures by a high frequency induction heater. This resistanceheats just the edges which are then brought together and loading applied to form a weld, without any addition of filler metal to the weld.

FatigueThe failure of materials by fluctuating or repeated stresses having a maximum value below the materials tensile strength. Fatigue failures often occur at loads which would not cause permanent damage if applied statically. The fracture process is usually progressive, i.e. it takes place over a number of load cycles.

FerriteIron or solid solution alloy of iron which has a bodycentred cubic crystallographic structure. In steels, the solute is generally carbon. Carbon has a very low solubility in ferrite, being only some 0.02 wt-%.

Ferritic Stainless SteelsThese are low carbon steels, usually that contain between 16 and 30% chromium. They are rarely used downhole.

Grain SizeMetals are generally crystalline materials, with the individual crystals known as grains. When a material is cooled slowly from a high temperature, e.g. in casting, it will generally have a coarse grain size. The grain size can be reduced by alloying additions (grain refiners, e.g. aluminium additions to steel), hot working and/or heat treatments (such as annealing). A smaller grain size will normally lead to greater strength, higher ductility and better toughness.

HardnessThe hardness of a metal is approximately related to its tensile strength. Therefore, hardness measurements can be used as a convenient non-destructive inspection technique. The hardness of a metal is often measured by an indentation test. In the indentation tests, a hard point is pressed into the material under a known load, the hardness is then judged from the size of the indentation. The most common types of hardness tests are:

The Brinell (HB) test in which a small hardened steel ball is used as the indenter and the diameter of the indent is measured

The Vickers (HV) test in which a pyramidal diamond indenter is used and the size of the indent is measured across the corners

The Rockwell (HRC) test in which a conical diamond indenter is used and the depth of the indent is measured

Tables to convert between these different test results are contained in ASTM E140.

HardenabilityThe relative ability of a ferrous alloy to form martensite when quenched. Hardenability is commonly measured as the distance below the surface at which the material exhibits a predetermined hardness (e.g. 50HRC).

Iron CarbideA compound of iron and carbon, e.g. Fe3C (cementite). When a steel is cooled from high temperatures the solubility of carbon decreases. The carbon that is thus pushed out of solution reacts with iron to form iron carbides. Carbon steels often contain a proportion of iron carbide as a result of the very low solubility of carbon in ferrite.

MartensiteIf steels are cooled rapidly there is insufficient time for the carbon to be pushed out of solution to produce large carbide particles/platelets. Therefore, a metastable transitional constituent is produced known as martensite. This transformation product is very hard/strong but very brittle. In most cases it is necessary to reintroduce some ductility by tempering.

Martensitic Stainless Steels

A group of hardenable stainless steels containing from 11 to 14% chromium and 0.15 to 0.45% carbon. These steels harden readily on air cooling from about 950C (1750F). It is usually necessary to reintroduce some ductility by tempering.

MonelA non-magnetic alloy containing nickel and copper. In the past this material was commonly used for non-magnetic drill collars (NMDC). Hence, NMDCs are also known as Monel collars. However, this material has been superseded for NMDCs by highly alloyed austenitic stainless steels, beryllium-copper alloys, etc.

Stainless SteelA corrosion-resistant type alloy steel which contains a minimum of 12% chromium. Chromium is the major element that confers upon the steel an ability to resist corrosion. This effect is attributed to the formation of a thin protective oxide on the metal surface. Corrosion resistance can be increased by the addition of other alloying elements, e.g. nickel, molybdenum, copper. The main types of stainless steel are austenitic, ferritic, martensitic, duplex and precipitation hardening.

Stress Relief Heat Treatment

A heat treatment designed to reduce internal stresses in metals that have been induced by casting, quenching, welding, cold working, etc. The metal is soaked at a suitable temperature for sufficient time to allow readjustments in the stresses, then slow cooled. Stress relief does not normally involve any structural changes within the steel.

TemperingA heat treatment to which steels, especially low-alloy steels, are subjected in order to produce changes in the mechanical properties and structure. This process often follows quenching, which produces a steel that is often too hard and too brittle to be of practical use. In tempering, the steel is heated to a suitable temperature at which structural changes will occur which reduce hardness (strength) and increase toughness. This is followed by cooling at a suitable rate. When Martensite is tempered, it gradually decomposes, with iron carbide being ejected from the solid solution. This is often called Tempered Martensite. The result of full tempering is a structure consisting of ferrite in which the iron carbide is dispersed as fine particles.

Tensile StrengthAlso known as the ultimate tensile strength (UTS), this is the maximum load sustained by a test piece during a tensile test divided by its original cross-sectional area.

ToughnessThe ability of a material to absorb energy and deform plastically before fracturing. One method of measuring this is the Charpy test. However, this is an imprecise measure as it can only indicate the energy absorbed at a particular temperature with a particular notch shape when subjected to impact or sudden loading. The threshold stress intensity factor (KIC) is a more accurate measurement, as this can quantify the critical stress for a crack to grow in a manner that accounts for the geometry of both the crack and the component. However, this is a difficult factor to quantify, so Charpy testing is normally used as a routine quality control measure.

Yield StrengthThe stress at which the material first exhibits a deviation from elastic behaviour (stress proportional to strain), i.e. the onset of plastic deformation.

Youngs ModulusA constant defined by the ratio of stress to the corresponding strain in the elastic region of a tensile test. As such it is a measure of the rigidity of a metal. It is a material characteristic that is independent of strength level or heat treatment.

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