• HADFIELD STEEL

    A manganese steel invented in 1882 by Robert A. Hadfield in Sheffield, England. It has an austenitic structure and an approximate analysis of 12.5% Mn, 1.2% C. Patents granted to Hadfield in 1882-85 covered alloys from 7 to 30% manganese. The steel was first made in the United States in 1892.

    Hadfield steel has certain characteristics which make it very useful. For example, it is work hardening. The metal is relatively soft and very tough after quenching in cold water after it is removed from the furnace. Hardness and toughness continue to increase as items made from this steel are impacted by repeated blows during service. See MANGANESE STEEL. 

  • HALF-LIFE

    The time required for a radioactive substance to decay to half its original value. Radioactive materials are used in radiographic inspection of welds. 

  • HALOGEN CONTAMINATION

    The presence of halides, particularly chlorides, has resulted in numerous in-service cracking failures of insulation-covered 18-8 austenitic stainless steels. These failures were first discovered in thermally insulated piping in petrochemical plants which had been built in the 1940s. The elimination of all halide sources during welding and installation, and the prevention of halide contamination during subsequent service, have proven to be extremely difficult. Some failures have been noted even in the presence of very low levels (10 ppm) of chlorides.

    These failures have been associated with stress corrosion cracking (SCC), an electrochemical reaction, which produces a fine network of transgranular cracks on the surface of the insulation-covered 18-8 stainless steel. Depending on conditions, failure by SCC may occur in as little as a few days or weeks.

    Four conditions are necessary for this SCC to develop:

    (1) An 18-8 austenitic stainless steel (such as 304, 304L, 316,316L, 317,321,347.

    (2) The presence of halides (particularly the chlorides)

    (3) The presence of tensile stresses (elastic or plastic, residual or applied).

    (4) The presence of an electrolyte (water). During SCC, the halide ions dissolve the passive protection layer on the 18-8 stainless steels; localized corrosion cells then become active.

    Austenitic stainless steels with higher nickel, chromium, and molybdenum contents have been developed for enhanced resistance to the SCC problem which has plagued the insulation-covered 18-8 stainless steels.

    Among the potential trouble-makers are the inks of several types of metal marking pens with high available halogen content, as well as perspiration from the worker’s hands. Clean cotton gloves should be worn when working with stainless steel. 

  • HAMMER WELDING

    A nonstandard term for forge welding and cold welding. 

  • HAMMERING, Resistance Spot Welding

    Excessive electrode impact on the surface of the workpiece at the start of the welding cycle. 

  • HAND SHIELD

    A protective device used in arc welding, arc cutting and thermal spraying to shield the eyes, face and neck. It is equipped with a filter glass lens and is designed to be held by hand. 

  • HARD CRACKING

    A condition which may develop in the coarse grain structure of the heat-affected zone of alloy steels, but which does not occur in mild steel. It is attributed to the effect of dissolved hydrogen released from austenite as it transforms. It can be avoided in alloy steels by preheating or by using low-hydrogen electrodes, or both. 

  • HARD SETTING

    A nonstandard term for the application of diamond- substitute inserts to wearing surfaces, using the oxyacetylene process with a welding rod of a softer material. See HARDFACING. 

  • HARD SOLDER

    A nonstandard term for brazing filler metal. See BRAZING. 

  • HARD SURFACING

    A nonstandard term for HARDFACING. 

  • HARDENABILITY

    The relative ability of a steel to form martensite when quenched from a temperature above the upper critical temperature. Hardenability is commonly measured by the Jominy (end-quench) test, in which the distance is measured from the quenched end to the point where nonmartensitic transformation occurs. See JOMINY TEST. 

  • HARDENING

    An action which induces hardness. Hardening is a term describing the heating and quenching of certain iron-base alloys from a temperature either within or above the critical temperature range. 

  • HARDFACING

    A surfacing variation in which surfacing material is deposited to reduce wear. See BUILDUP, BUTTERING, and CLADDING.

    Hardfacing is the application of a hard, wear-resistant material to the surface of a workpiece by welding or spraying, or allied welding processes, to reduce wear or loss of material by abrasion, impact, erosion, galling and cavitation.  The stipulation that the surface be modified by welding, spraying or allied welding processes excludes the use of heat treatment or surface modification processes such as flame hardening, nitriding, or ion implantation as a hardfacing process.  The stipulation that the surface be applied for the main purpose of reducing wear excludes the application of materials primarily used for prevention or control of corrosion or high-temperature scaling. Corrosion and high-temperature scaling may, however, have a major effect on the wear rate, and for this reason may become a significant factor in selection of materials for hardfacing.  Hardfacing applications for wear control range from very severe abrasive wear service, such as rock crushing and pulverizing, to minute mechanical applications that require minimization of metal-to-metal wear, such as control valves where 0.05 mm (0.002 in.) of wear is intolerable. Hardfacing is used for controlling abrasive wear on mill hammers, digging tools, extrusion screws. cutting shears, parts of earthmoving equipment, ball mills, and crusher parts. It is also used to control wear of unlubricated or poorly lubricated metal-to-metal sliding contacts such as control valves, undercarriage parts of tractors and shovels, and high-performance bearings.

    Hardfacing Materials

    Hardfacing materials include a wide variety of alloys, ceramics, and combinations of these materials. Conventional hardfacing materials are steels or low-alloy ferrous materials, chromium white irons or high alloy ferrous materials, carbides, nickel-base alloys, or cobalt-base alloys. A few copper-base alloys are sometimes used for hardfacing applications, but for the most part, hardfacing alloys are either iron, nickel or cobalt base.

    The microstructure of hardfacing alloys generally consists of hard-phase precipitates such as borides, carbides, or inter-metallics bound in a softer iron, nickel or cobalt-base alloy matrix.

    Cobalt-Base Alloys- The alloys listed in Table H-1 that contain 2.5% C have more than 30% by volume total carbides, which results in extremely high abrasion resistance. The microstructure of the Co-30, Cr-12, W-2.5, C alloy, sometimes referred to as Alloy No. 1, has a large volume fraction of carbides. As the carbon content is increased, the volume fraction of the matrix is decreased, and the impact resistance, weldability and machinability are also decreased. Thus, the improvement in abrasive wear resistance is gained at the expense of other properties that may be more desirable.

    Nickel-Base Alloys- The commercially available nickel-base hardfacing alloys can be divided into three groups: boride-containing alloys, carbide-containing alloys, and Laves phase-containing alloys. The compositions of some typical nickel-base hardfacing alloys are listed in Table H-2.

    The boride-containing nickel-base alloys are commercially produced as spray-and-fuse powders. These alloys are available from most manufacturers of hardfacing products under various trade names and in a variety of forms, such as bare cast rod, tube wires, and powders for plasma spraying. This group of alloys is primarily composed of Ni-Cr-B-Si-C. Usually, the boron content ranges from 1.5% to 3.5%, depending on chromium content, which varies from 0 to 15%. The higher chromium alloys generally contain a large amount of boron, which forms very hard chromium borides with hardness of approximately 1800 DPH (kg/mm2). Other borides high in nickel and with lower melting points are also present to facilitate fusing.

    The abrasion resistance of these alloys is a function of the amount of hard borides present. Alloys containing large amounts of boron such as Ni-14, Cr-4, Si-3.4, B-0.75, C are extremely resistant to abrasion, but have poor impact resistance. Because most of the boride- containing nickel-base alloys contain only small amounts of solid-solution strengtheners, considerable loss of room-temperature hardness occurs at elevated temperatures. 

    Carbides- The quantity of carbides used for hardfacing applications is small compared with iron-base hardfacing alloys, but carbides are extremely important for severe conditions presented by some abrasion and cutting applications. Historically, tungsten-base carbides were used exclusively for hardfacing applications. Recently, however, carbides of other elements, such as titanium, molybdenum, tantalum, vanadium and chromium have proven to be useful in many hardfacing applications

    The widespread use of carbides for hardfacing is primarily based on the general belief that all carbides, due to their high hardness, resist fracture and fragmentation as well as abrasion, especially under high-stress applications. In reality, the resistance of carbide composites is a function of the abrasion resistance of the matrix. While the various carbides have high hardness values, they unfortunately do not have resistance to crushing force, i.e., fracture and fragmentation. Carbides should not be selected based solely on hardness value. For comparison, Table H-4 lists the hardness of various carbides and other selected materials.

     

    Copper-Base Alloys- The copper-base hardfacing alloys are similar to bronzes and are used in applications where copper-base bearing materials are normally employed as homogeneous parts. It is often more economical to apply copper-base hardfacing alloys as overlays on less expensive base metals such as low-carbon steels.

    The properties of copper-base hardfacing alloys are similar to the properties of corresponding bronzes. Copper-base hardfacing alloys are used for applications where resistance to corrosion, cavitation erosion and metal-to-metal wear is required, as in bearing materials. Copper-base hardfacing alloys have poor resistance to corrosion by sulfur compounds, abrasive wear and elevated-temperature creep. They are not as hard as all the classes of alloys previously discussed, and are not easily welded.

    Hardfacing Alloy Selection

    Hardfacing alloy selection is guided primarily by wear and cost considerations. However, other manufacturing and environmental factors must also be considered, such as base metal, deposition process, and impact, corrosion, oxidation and thermal requirements. Usually, the hardfacing process dictates the hardfacing or filler-metal product form.

    Hardfacing alloys are usually available as bare rod, flux-coated rod, spooled solid wires, spooled tube wires (with and without flux), or powders. Table H-5 lists various welding processes, heat sources, and the proper forms of consumables for each process. In general, the impact resistance of hardfacing alloys decreases as the carbide content increases. As a result, in situations where a combination of impact and abrasion resistance is desired, a compromise between the two must be made. Where impact resistance is extremely important, austenitic manganese steels are used to build up worn parts.

    Hardfacing Process Selection

    Hardfacing process selection, like hardfacing alloy selection, depends on the engineering application or service performance requirements. Other technical factors involved in hardfacing process selection include (but are not limited to) hardfacing property and quality requirements, physical characteristics of the workpiece, metallurgical properties of the base metal, form and composition of the hardfacing alloy, and welder skill. Cost considerations are often the determining factor in the final process selection.

    Traditionally, hardfacing has been limited, by definition, to welding processes. However, this definition has been expanded to include thermal spraying (THSP) as a hardfacing process. Frequently the first consideration in hardfacing process selection is to determine if welding processes or THSP processes are preferred or required. As a rule, welding processes are preferred for hardfacing applications requiring dense, relatively thick coatings with high bond strengths between the hardfacing and the workpiece.

    Thermal spraying processes, on the other hand, are preferred for hardfacing applications requiring thin, hard coatings applied with minimum thermal distortion of the workpiece. Source: ASM International; Metals Handbook, Desk Edition; ASM International. 1985. 

     

  • HARDNESS

    The resistance of a material to plastic flow, most often measured by indentation by a penetrator under an impressed load. Additionally, hardness may refer to the resistance to machining, abrasion, or scratching. See HARDNESS TESTING. 

  • HARDNESS TESTING

    Hardness tests are used to evaluate welds, either alone or to complement information on other test results. The Rockwell, Brinell, Vickers, and Knoop tests are indentation hardness tests that measure the area or depth of indentation under load to determine the hardness. The indentations are made with testing machines selected on the basis of specimen size, form, and purpose of the hardness measurement. Indentation hardness testing is a complex measurement because of the different degrees of work hardening that occur in metals and the influence of the indenter used.

    In the Brinell, Vickers, and hoop tests, the area of the indentation is measured to determine hardness. Rockwell hardness testing relates hardness to the depth of indentation under load. 

    Rockwell Hardness (HR)- The Rockwell hardness test has become the most widely used method for determining hardness because it provides scales that can accommodate specimens of a wide variety of metals in a wide variety of sizes and shapes. The Rockwell hardness test is simple to perform; the hardness number is conveniently read directly on the testing machine, and the testing can be automated if required.

    The procedure involves initial application of a minor seating load to the indenter to establish a zero datum position. A diamond-tipped indenter with a sphero-conical shape is used for hard metals, and a small hardened steel ball of prescribed size is used for softer metals. Both the minor load and the major load can be selected, depending on specimen requirements. More than a dozen scales of hardness numbers have been tabulated; each is designated by a letter of the alphabet. These basic scales are supplemented by additional scales that provide modified conditions to compensate for specimen form (eg, curvature) and approximate level of hardness. Rockwell hardness numbers should always be quoted with a scale symbol, which indicates the kind of indenter, major load, and other testing conditions.

    Three Rockwell scales are most commonly used for measuring the hardness of steels:

    (1) C Scale, which uses a sphero-conical indenter which applies a 150kg major load

    (2) B Scale, which uses a ball indenter (usually 1.588 mm [1/16 in.] diameter) and a major load of 100 kg (these conditions can be adjusted by an established correction factor)

    (3) N Scale, which encompasses many established conditions for superficial hardness testing.

    Brinell Hardness (HB). The Brinell method for testing hardness, like the Rockwell scale, has a long history of applications and is commonly used in many metal working plants. The Brinell test is used to monitor mechanical properties in metal articles of substantial size, such as bars, beams, or plates. The Brinell scale is based on the impression made in a flat surface by a hardened steel ball 10 mm (.39 in.) in diameter,

    when driven into the metal at a force of 3000 kg (6600 lb.). The 30-second test time ensures that plastic flow of the metal surrounding the indentation has ceased. A standard procedure is used to measure the diameter of the indentation and to compute the Brinell hardness (HB) number, using an equation that relates load applied, ball diameter, and indentation diameter to the hardness number. (Computation is seldom needed, since most test results are available in tabular form).

    Standards for testing are set forth in ASTM E10, Brinell Hardness of Metallic Materials, and ASTM E 370, Mechanical Testing of Steel Products.

    Vickers Hardness Test (HV)- The indenter, a square-based diamond pyramid with a 136" included angle, is used with a variety of loads in the range of 1 kg (2.2 lb) to 120kg (264 lb). In this microhardness test, impressions can be closely spaced and depth of penetration can be very small.

    A standard method for this test is provided in ASTM E92, Vickers Hardness of Metallic Materials.

    Knoop Hardness Test (HK)- A very small indenter, a rhombohedral-based diamond with edge angles of 172"30 and 130", is used with a variety of loads, usually under 1 kg (2.2 lb). The impression has one long and one short diagonal. Impressions can be very closely spaced and the depth of penetration can be extremely small.

    Microhardness Testing

    Microhardness tests can be performed with a number of instruments that use a very small indenter and a very light, precise load to make an indentation in a polished surface. The resulting indentation is measured by microscope. A polished and etched metallographic specimen is frequently used to allow hardness determinations on individual phases or constituents in the microstructure. By using an indenter with a test load in the range of 1 to 1000 g, the indentation can be confined to a single grain in the microstructure. Standards for microhardness testing using the Knoop and the Vickers instruments are covered in ASTM E384, Microhardness of Materials.

    Scleroscope Testing Equipment

    The Shore Scleroscope is a hardness testing machine which consists of a vertical glass tube in which a small cylinder, or hammer, with a very hard point slides freely. This hammer weighs 2.5 grams, and is allowed to fall 25 cm on to the sample to be tested. The distance which it rebounds, measured on a scale on the glass tube, constitutes the hardness.  The scale is divided into 140 parts, each part representing a degree of hardness. As examples of this scale, the hardness of glass is 130; the hardest steel is 110; mild steel is in the range of 26 to 30, and cast gray iron is 39.

    Comparison of Scales

    The relationship among the several hardness scales is presented in Table H-6, showing the appropriate equivalent hardness values for steels.

    Brinell Tensile Strength

    The Brinell hardness of steel will give a fairly accurate indication of the tensile strength of the material. It has been found that by correlating the Brinell hardness numbers and the tensile strength of various steels in lb/ in., the tensile strength of a given steel is approximately 500 times its Brinell hardness number. In determining the tensile strength by the use of this rough check it has been found that as a rule, the tensile strength will be slightly low for hardness below 200 HB and above 400 HB. Between the two figures the indicated tensile strength is slightly above the actual strength. 

  • HARTFORD TEST

    The Hartford test involves an inspection of welds for insurance purposes. It is primarily a procedure qualification test in which sample welds are made using the same material, same equipment and same type of welding wire which are to be used on the job under construction. Even when this combination has been tested and approved, it is necessary before actual work can be started to satisfy the Hartford inspector that the welding operator who is to do the work is capable of producing welds equal in quality to those obtained in the procedure qualification test.

    This qualification test remains valid for the individual welder only as long as he continues to work in the same shop with the same equipment and the same welding wire. If the manufacturer should wish to change any of the details of the welding procedure, the welder may be required to repeat the entire qualification program. 

  • HEAT BALANCE

    The various material, joint, and welding conditions that determine the welding heat pattern in the joint. 

  • HEAT INPUT

    The energy supplied by the welding arc to the work piece. 

  • HEAT OF FUSION

    The heat given off during the freezing (cooling and solidification) of a metal or alloy, or absorbed during the melting; sometimes called the heat of solidification. It is expressed in calories per gram. In the case of alloys, the processes of melting and freezing are complex and usually occur over a range of temperatures rather than at a single temperature. See METALLURGY. 

  • HEAT TIME

    The duration of each current impulse in multiple impulse welding, resistance seam welding or projection welding. See Figure H-3. 

  • HEAT TREATMENT

    The post-welding introduction of heat to the weldment, to remove or improve conditions brought about by the heat of welding. Reduction in grain size, surface hardening, annealing or normalizing, or stress relief are all within the capability of correct heat treatment.

    In most shops, post weld heat treat (PWHT) is accomplished in a heat treat furnace with controlled temperature modes allowing for temperature increase, hold-at-soaking temperature, and controlled cooling rate. For small weldment made by oxyacetylene welding, the torch flame can be used for heat treating. For field projects, two heating processes are available: exothermic and electrical resistance. Each method has advantages as well as limitations for use as a heat treating process.

    Exothermic- Exothermic materials are special combustible materials which burn under controlled conditions. They are commercially available in molded shapes and flexible lengths that can be stored and cut to fit as needed. The exothermic material is wrapped around the weldment, such as a pipe joint, and wired in place. Then a flame is applied to the material and it burns rapidly, giving off large quantities of heat. When the temperature reaches a predetermined point, determined by experimentation, the exothermic material is completely consumed. Cooling of the joint is controlled by the thickness of the insulation backing on the form. After cooling, the wrapping wires are cut and the material is removed from the joint. In recent years, however, the use of exothermics for PWHT has fallen off sharply due to environmental and thermal control considerations.

    Electrical Resistance- Post weld heat treatment using resistance heating involves wrapping the joint with a number of resistance heaters. Advanced electrical resistance systems with automatic controllers make it possible to heat treat several weldments simultaneously. 

  • HEAT-AFFECTED ZONE (HAZ)

    The portion of the base metal whose microstructure or mechanical properties have been altered by the heat of welding, brazing, soldering or thermal cutting. See Figure H-2. See also METALLURGY. 

  • HEAT-AFFECTED ZONE CRACK

    A crack in the heat-affected zone of the weldment. 

  • HEATING GATE

    The opening in the thermite mold through which the parts to be welded are preheated. See also THERMITE WELDING. 

  • HEATING TORCH

    A device for directing the heating flame produced by the controlled combustion of fuel gases.