STAINLESS STEEL INFORMATION SERIES PULL FORGING OF OUT STAINLESS STEEL PART 10

he art of forging has existed from prehistory, when man first TABLE 1: The Nominal Composition of Grades of Stainless Steel began to work with metal. Until T MARTENSITIC STAINLESS STEELS AS TYPIFIED BY THE AISI 400 SERIES comparatively recently it was, with few GRADE C%(1) Cr% Ni% Mo% Other % and Remarks exceptions, the only method of shaping 410 0,15 11,5-13,5 solid metal into forms such as swords, 416 0,15 12-14 S0,15 P 0,06 Mn 1,25 / Free-Machining axes, eating and cooking utensils, 416Se 0,15 12-14 S 0,06 P0,06 Mn 1,25 Se0,15 / Free Mach 420 >0,15 12-14 coinage and jewellery. 431 0,20 15-17 1,3-2,5 Forging may be defined as the process 440A 0,60-0,75 16-18 0,75 max of shaping metal by the controlled 440B 0,75-0,95 16-18 0,75 max amount and direction of plastic 440C 0,95-1,20 16-18 0,75 max deformation using impact or pressure. FERRITIC STAINLESS STEELS AS TYPIFIED BY THE AISI 400 SERIES It is usually done hot, but cold GRADE C%(1) Cr% Ni% Mo% Other % and Remarks forging is also carried out. 430 0,12 14- 18 430F 0,12 14- 18 S0.15 P 0,06 Mn1,25 / Free-Mach The advantages of forging include: 446 0,2 23-27 N 0,25 max (•) 3CR12 0,03 11-12 • Virtually all metals and alloys can be forged. AUSTENITIC STAINLESS STEELS AS TYPIFIED BY THE AISI 300 SERIES (1) • A wide range of sizes and shapes GRADE C% Cr% Ni% Mo% Other % and Remarks 301 0,15 16-18 6-8 can be produced. 302 0,15 17 19 8-10 • The forgings are of near net shape. 303 0,15 17- 19 8- 10 S0,15 P0,12 / Free-Machining • Excellent refinement of the 303Sc 0,15 17- 19 8- 10 S0,06 P0,12 SeO,15 / Free-Machining microstructure takes place, 304 0,08 18-20 8-12 particularly in that a continuous 304L 0,03 18-20 8- 12 309 0,20 22-24 12- 15 grain flow consistent with the shape 310 0,25 24-26 19 22 of the forging occurs, which results 316 0,08 16 18 10- 14 2-3 in enhanced mechanical properties. 316L 0,03 16- 18 10- 14 2-3 316F 0,06 18 13 2,25 S0,15 P0,13 Mn1,5 / Free-Machining In recent times, forging equipment has 317L 0,03 18-20 11-15 3-4 321 0,08 17- 19 9- 12 Ti (5 x % C) min / Stabilised increased in size and power, and precise 347 0,08 17- 19 9-13 Nb + Ta (10 x % C) min / Stabilised controllability has been developed. Forging may now be considered as an art DUPLEX STAINLESS STEEL TYPICALLY AVAILABLE AS PROPRIETARY GRADES GRADE C%(1) Cr% Ni% Mo% Other % and Remarks under scientific control. (•) 0,03 23 4 N0, 10 (•) 0,025 22 5,5 3,0 N0.15 STAINLESS STEEL (•) 0,025 18,5 4,7 2,7 Stainless steel is not a single material, (•) 0,03 22 6 2,5 N 0,05 Cu 1,75 but a family of different classifications PH STAINLESS STEEL TYPICALLY AVAILABLE AS PROPRIETARY GRADES and grades, based on alloying chromium GRADE C%(1) Cr% Ni% Mo% Other % and Remarks (Cr) with iron (Fe). A chromium content (•) 0,07 16 4 Cu 3,5 Nb+Ta 0,15-0,45 in excess of ±11 % Cr imparts to stainless (•) 0,05 12,5 8 2,5 Al 1,1 steel its natural built-in corrosion (•) 0,07 17 / Al 1,2 resistance due to the formation of an (•) 0,10 16,5 4,3 2,7 N 0,1 (•) 0,10 17 11 P0,3 extremely thin but continuous and stable Notes: (1) % max unless indicated as minimum or range chromium oxide film on its surface. (•) Indicates Proprietary Alloy - Nominal composition given This has been covered in detail in AII steels will contain normal amount of Si, P, S, except where specifically indicated C = Carbon; Cr = Chromium; Ni = Nickel; o - Molybdenum; Si - Silicon; S = Sulphur; previous modules in this series. P = Phosphorus; Mn = Manganese; Se - Selenium; Ti = Titanium; N = Nitrogen; Cu - Copper PRODUCTION OF STAINLESS STEEL desired final shape, and therefore can Different forging processes include: FORGINGS be highly dependent on the skill and • Drawing Out A great variety of stainless steel experience of the smith. • Upset Forging forgings of different size and shape • Much forging done in “Jobbing” • Upsetting are produced, ranging from medical forge plants use this process. • Ring Forging and Ring Rolling instruments weighing only a few grams • Die Forging to massive forged heads and cylindrical CLOSED DIE FORGING (CDF) • Piercing and Core Forging shells of nuclear pressure vessels which The final shape and size of the forging These forging processes are briefly weigh several hundred tons. is produced between dies into which the described below, but many variations Production is carried out by: impression (shape) has been machined. of the techniques used, and the number • The steelmakers, i.e. conversion • Preliminary shaping by the and sequence of operations employed of steel made in their own steel workpiece is often necessary to produce the required final forged plant; either as limited quantities of before the final shape is forged. component. Forging is still as much an forgings of large size required for - This is often carried out by ODF or art as a science and each smith has his specific applications, or as larger a combination of ODF and CDF. own techniques. quantities of standard forgings . • Single or repeated application of the • In-House Forge Plant, i.e. conversion forging force may be used dependent DRAWING OUT of purchased steel into forgings on the material being forged, the Increasing the length of the workpiece specifically for the company’s own forging equipment employed, and by reduction of the workpiece thickness range of products, and which will the complexity of the shape. as it is progressively fed between the usually be of large quantities falling • Higher forging forces are forging surfaces. within a narrow range of size, mass, required, which increase as • The workpiece is drawn out and shape. the degree of confinement on the square by frequently • “Jobbing” Forge Plant, i.e. conversion and complexity increases. rotating it through 90°. of purchased steel to meet a - This is due to the friction - Rounding up of round bar is only wide variety of individual user between the workpiece and done once the square cross section requirements, normally consisting the die, and the surface of the is that of the diameter. The comers of a small quantity of forgings of workpiece being chilled by the die. are first rounded and then frequent diverse size, mass, shape, and types - Die lubricants, which are usually applications of force of lower of steel (plain carbon, low alloy, tool mixtures of graphite and mineral intensity are given as the workpiece and die, stainless steels etc.). oils, should be judiciously used. is rotated through small angles. Excessive use may lead to the Improved size tolerance and surface FORGING EQUIPMENT carburisation of the surface layers finish is obtained if final forging is The equipment used to produce thereby decreasing the corrosion done between swage blocks. Dies forgings varies with respect to: resistance of some grades of can vary from being very simple to The forging processes which can be stainless steel. extremely complex, from small to carried out: • Die design, including size, draft large, and with respect to the grades • The method of applying the force. (the taper on the side walls of the of steel from which they are made. • The size. impression), the formation and • It is an Open Die Forging technique. The different types of forging accommodation of the flash, and • The commonly used forging equipment include: the location of the parting line equipment are steam hammers and • Hammers are important due to the greater hydraulic presses. • Presses high temperature strengths of (Note: The piston of steam hammers • Upsetters Stainless steels, and hence their used to be driven by steam; hence the name. • Forging Rolls higher resistance to metal flow. Compressed air is now normally employed, • Ring Rollers - A die used for the forging of plain but the term steam hammers is still used). • Swaging Machines carbon and low alloy steels may well • Common products include round, Forging Processes prove to be unsuitable for the forging of square, flat and hexagon bars; The shaping of the metal is normally the same component in stainless steel. stepped shafts. done using two different techniques: - The die life when forging • Open Die Forging stainless steels is much less than UPSET FORGING • Closed Die Forging would be obtained when forging This is compression of the workpiece In the production of some forgings a plain carbon and low alloy steels. along its longitudinal axis where its combination of both techniques may be - Dies can vary from being very length (height) is decreased and its used. simple to extremely complex, from diameter increased. small to large, and also to the grades It is a very severe forging process. OPEN DIE FORGING (ODF) of tool steel from which they are • The deformation is non- The forging is shaped between two made. homogeneous. The metal at and flat (or simply contoured) surfaces by the • Most of the high quantity repetitive near the centre is deformed to a great repeated application of the forging force production of identical forgings as extent. This forces the metal located (hammering or pressing). Manipulation done by Production and In-House. near the circumference outwards of the workpiece is needed to obtain the - Forge Plants is done using CDF. causing moderate deformation in these areas. The metal in contact RING FORGING AND RING ROLLING more of the forging processes. with the forging surfaces undergoes The initial forging process is upset • The commonly used forging little deformation. forging to produce a blank that is punched equipment is Single Action and • The metal which is to be upset must and pierced to make a central hole. Steam Hammers, Mechanical and therefore be of prime quality, having Hydraulic Presses. a homogeneous microstructure with RING FORGING • Common products include hooks, no central segregation and a defect The pierced blank is forged over a surgical and dental instruments, free surface. mandrel, rotating the blank by small offset shafts, crankshafts, conrods, The maximum length/height which amounts for repeated applications of valve bodies, pipe fittings, injection can be upset is limited. The length/ forging force. This progressively increases nozzles, turbine vanes, pump height (H):cross section (D) is dependent the outside diameter (OD) and decreases components, shackles, swivels, on the material being forged. the wall thickness (WT). This operation is turnbuckles, cable clamps. • For plain carbon and low alloy often referred to as Ringing-Up. steels H:D can be up to 3:1. • It is an Open Die Forging technique. PIERCING AND CORE FORGING - Due to the greater high • The commonly used forging Piercing is the forging of hollow temperature strength of stainless equipment is steam hammers and shaped components by forcing a punch steels H:D is limited to usually hydraulic presses. into the workpiece which is contained in 2:1, and a maximum of 2.7:1. • It is well suited to produce small a die. The metal flow can either be: - Attempting to upset forge longer quantities of different sizes of rings. • Radial, i.e. diameter increases but lengths will often result in buckling Bushes and sleeves can also be length remains the same. of the workpiece before any bulging produced. • Rising, i.e. diameter remains is attained. • Common products are rings, virtually constant but the length The commonly used forging bushes and sleeves which then find increases along the length of the equipment includes steam hammers, application as various components, punch. mechanical and hydraulic presses. e.g. flanges, hubs, gear rings, gear In piercing, the OD of the workpiece Common products include discs, blanks, bearing housings, bearing is not usually contoured. blind flanges, slip-on flanges weld neck rings, trunions, sockets. Core forging increases the OD of the flanges, gear blanks, tube plates. workpiece and also forges the metal into • Upset forging is often the initial RING ROLLING the containing die cavity to produce a forging process used to form the This is not truly a forging process, but contoured OD on the workpiece. blank required for the forging of it is being increasingly adopted by forge • Both are Closed Die Forging rings. shops to convert the forged and pierced techniques. blank to larger rings of the required final • The commonly used forging UPSETTING dimensions. High production rates (cf equipment used is Mechanical and This is compression forging in which ring forging) are attained. Hydraulic Presses. only a portion (short length) of the • Rolling of the ring as a continuous • Products include cylindrical and workpiece is upset; either at the end of, “flat bar” takes place, the OD conical components, pipe fittings, or at a location within the length of the increasing and the WT decreasing as valve bodies, cups, sleeves. workpiece. the idler roll is brought closer to the The factors as outlined in Upset drive roll. The width is controlled by STARTING STOCK FOR FORGING Forging apply: edging rolls. The starting stock for forging may be • The upset is general performed with • The range of sizes which can • Cast Ingot a single application of the forging be produced depends on the • Continuously Cast Strand/Billet force. In some cases a preform upset configuration of the equipment. • Semi finished Products (Semis) such may need to be performed in one set - The width that can be produced as rolled billet of dies, and then subsequently upset is dependent on the length of • Finished product such as Bar to final shape/size in another set of the idler and drive roll. The dies. ability to produce bushes and CAST INGOT • For small components the upsetting sleeves may therefore be limited. The forging of cast ingot is usually may be done cold, i.e. without any - As the WT increases the process is only carried out by the steelmakers heating of the workpiece (e.g. the progressively less suitable, as the force themselves to produce either forgings, heads of screws and bolts). and dimensional control which the or semi- finished product for sale as re- • Only CDF technique is used. The edging rolls can exert may be limited. forging stock. forging equipment used is Upsetters, - Special profiles can be produced by Prior to forging, the cast ingot must or special purpose mechanical/ suitably shaped contours machined be surface conditioned (usually by heavy hydraulic presses which operate in either the drive or idler rolls. duty grinders) to remove surface defects, in the horizontal plane. Common and cropped to eliminate any areas of products include fasteners (screws, DIE FORGING shrinkage (pipe) and gross segregation. hex head bolts, eye bolts), valve This is the forging of the component Breakdown of ingot is done by stems, spindles, hub, gear blanks, in fully closed impression dies (i.e. CDF). drawing out. The initial forging is by stub axles, small rings and flanges Pre-shaping of the workpiece may be light reduction to break down the coarse (by first heading and then piercing). done by ODF and/or CDF using one or cast granular structure, and to effect initial refinement and consolidation. reduction to ensure refinement/ surface layers and the cooler interior. Once this stage of fragility has been consolidation, and condition the Internal cracking may result, termed passed subsequent breakdown with surface, so that it may be successfully “klinking”, which will open and heavier reductions can be accomplished. converted provided the necessary progress to the surface in the • Complete consolidation and precautions and good working subsequent forging operation. (This refinement of the internal structure practices are applied during heating, is more prone to occur in heating of will require a reduction of cross- forging and post forging operations. cast material). sectional area of between 4 and 6:1, • The undesirable high temperature depending on the melting, refining FINISHED PRODUCT such as BAR crystal structures (such as delta and ingot pouring techniques which Finished product (usually round bar) ferrite) may form in the surface have been used. is used for forging operations in which layers of some grades of Stainless • The upset forging of cast ingot may stock of highest integrity and accurate steel. As this crystal structure has be considered as virtually impossible. size is required (such as upsetting, die lower transverse ductility, cracking Failure during forging will usually forging where no blocking is carried out, within the surface can occur in the result due to the unrefined and core forging). initial stages of the subsequent unconsolidated structure being • The bar supplied is usually rough forging operation. unable to accommodate the severe machined to fairly close size • Gross overheating of the surface and non-homogeneous deformation tolerances so that the exact volume layers may occur, resulting in which takes place. can be cut for the required forging, “burning” (incipient melting) taking or the exact length for the required place at the grain boundaries. When CONTINUOUSLY CAST BILLET upset in upsetting can be indexed. the forging force is applied the Breakdown is done by drawing out. - Rough machining will also ensure workpiece cracks or disintegrates Continuously cast strand/billet has an a defect free surface. along the grain boundaries which internal structure which is superior to • When ordering such material, it is are “lubricated” by molten metal. that of cast ingot. still advisable to state on the order” The following additional factors • Therefore, for complete BAR FOR REFORGING PURPOSE”, should also be borne in mind. consolidation and refinement of the and also to stipulate the forging • The workpiece should be frequently internal structure less reductions process which is to be employed. turned during heating. Otherwise in cross-sectional area is needed. the area of the workpiece in contact - Continuously cast billet should HEATING FOR FORGING with the hearth will take too long not be taken and simply rounded • Heating requirements which are to, or may not, attain the correct up to produce forged bar of suitable for plain carbon and low forging temperature. - Placing the similar cross section. Inferior alloy steels are NOT applicable workpiece on bearer bars assist in mechanical properties will result. for Stainless steels. Care must be more uniform heating. - If upset forging is to be done, prior taken to ensure that the appropriate • The atmosphere should be consolidation and refinement must heating parameters are applied. controlled to be slightly oxidising be effected (reduction of cross- Shortcomings will lead to a high in nature. If too highly oxidising, Cr sectional area by ± 3:1). failure rates during forging. depletion may occur. If reducing, C Stainless steels have a low Coefficient pick-up (carburisation) can occur in SEMIFINISHED PRODUCTS such of Thermal Conductivity (particularly the surface layers, with a reduction as ROLLED BILLET the austenitic stainless steels) compared of the corrosion resistance. Semi-finished products (billets) are to plain carbon and low alloy steels. • No flame impingement onto the bought from steelmakers by the forging Therefore, longer times are needed to workpiece should occur. industry for subsequent forging into attain the required forging temperature • Refinement of the microstructure finished products. uniformly throughout the workpiece. which results from the heating of the Production of billet is either by • Typical heating time from ambient workpiece to the high temperatures rolling (in cogging/blooming mills) or to the required forging temperature necessary for forging is an important by forging the original cast material to can vary from 40-90 minutes per objective of the forging process. The the smaller cross-sectional sizes of billet. 25mm of nominal section thickness. temperature to which the workpiece • Billet which has been forged tends to - Larger cross-sections will is heated depends on the amount and have a better internal consolidation require longer heating times rate of the deformation which will and refinement of the microstructure per 25mm of nominal thickness be effected. The higher temperatures than does rolled billet (especially in than smaller cross sections. should only be used if a large the larger sizes of >±200mm). - The type of furnace will also have amount of deformation is required. In ordering such billet, either by an effect. If more than one excursion to the forge shops or by steel merchants/ Because of these longer heating times forge is required to accomplish the stockists, it should be specifically stated the temptation may be to run the furnaces necessary shaping of the workpiece, in the order “BILLET FOR REFORGING with a relatively high degree of superheat. lower initial and reheat temperatures PURPOSES”, and if upset forging is to be This should not be done because: may be needed as the amount of effected this should also be stipulated. • Rapid heating of the surface layers deformation during each excursion is • The steelmaker should therefore occurs which results in a differential usually of a lesser amount. produce such material with sufficient in expansion between the hot • High rates of deformation during forging can maintain or even raise ferrite), and possibly even incipient module on Stainless Steel Welding). the temperature of the workpiece. If grain boundary melting, both of When heating martensitic grades for this is to be the case the temperature which seriously impair the forging forging: to which the workpiece is heated properties. In addition, the required • It is advisable to preheat certain should be modified accordingly. refinement will not take place (or grades (particularly if the workpiece • Soaking at the forging temperature be non-uniform) which will detract has a large cross-sectional thickness) must be avoided as the longer times from the properties of the forging. to an intermediate temperature, and will cause high temperature induced • Forging should be finished at as equalise the temperature throughout changes to the microstructure, which low a temperature as possible to before charging the workpiece into are more difficult to refine or may maximise the refinement of the the furnace to heat to the higher cause forging difficulties. microstructure. temperatures required for forging. • If more than one forging operation is • Forging of martensitics must not be COOLING AFTER FORGING required the amount of deformation carried out at temperatures below Stainless steel must be suitably cooled which occurs in each should not be that at which crystal structure change after forging dependent on the chemical on a random basis, but be planned to occurs, (±810°C termed the altotropic composition (classification/type) and on optimise the grain refinement. transformation temperature). the size/section thickness. • The free-machining grades of However, forging is usually finished Stainless steel have additions at the higher temperatures as given FORGING of Sulphur (S) or Selenium (Se). below because deformation becomes Forging parameters vary for each - These both impair the forging increasingly more difficult at lower classification/type of stainless steel, and properties due to the formation of temperatures. sometimes (to a lesser degree) for the stringers within the microstructure. Cooling after forging is a very different grades in the same classification. Se has a lower tendency to do so, important factor in the forging of However, in general, the following and therefore is the preferred free- martensitics. Due to their hardenability, apply in the forging of all stainless steels. machining addition in Stainless especially the high C grades, they must • Due to their greater strength at high steels which are to be forged. be carefully cooled. temperatures Stainless steels need • The suggested forging temperature • As a minimum requirement the more force (forging pressure) or a range for the different grades of forging must be held in a dry warm larger number of blows to cause Stainless steel are given in the insulating material immediately the required amount of plastic sections below. - Reference to after forging is completed, and deformation (cf plain carbon and alternative sources of forging data allowed to cool to below 550ºC low alloy steels). will most certainly give different after which it can be removed and • If surface defects/tears occur the values. Therefore, the suggested allowed to cool in air. It is preferable forging operation should be stopped temperatures given should be used to cool the higher Cr and high C immediately otherwise they will as a guide until experience dictates grades (e.g. Grades 420; 440A,B,C) progressively get larger/deeper. The otherwise. by slow furnace cooling at ±25°C/ defect may be then dressed out of hr. Such slow continuous cooling of the workpiece. - Either whilst hot by MARTENSITIC STAINLESS STEELS the high C grades can result in an gouging/ cutting (usually confined This classification of stainless steel unacceptable excessive formation to small defects only). - Or by is extensively used to produce forged of grain boundary carbides. To grinding after cooling the workpiece components due to the properties overcome this, a modified cooling in the appropriate manner (strength/hardness, abrasion resistance) sequence is required - air cool to dependent on the grade of stainless which can be developed by heat ±250°C (taking great care to ensure steel. The heating of the surface treatment. that lower temperatures are not layers by heavy localised grinding The main factor in forging these steels attained), and then immediately is high and care must be taken with is to exercise close control of the maximum “tempering” at 650°-680°C before the martensitic grades that this forging temperature attained (both in the finally allowing to cool freely in air does not cause heat checking of the heating and during the forging of the to ambient temperature. ground surface which would again workpiece) due to their tendency to form • For complex forgings which have initiate defects during subsequent delta ferrite at temperatures in the range large and/or sudden changes in forging. (Preheating or thorough of ±1100°-1250°C. thickness of cross-section it may flooded cooling while grinding are • A composition which gives a high prove necessary to carry out a full suggested in order to prevent this). Chromium Equivalent will result in annealing process immediately after • Intense and rapid deformation can delta ferrite formation at the lower forging. cause an increase in the temperature temperatures within this range, A further aspect related to cooling of the workpiece. Further, if this and correspondingly the lower is that during the actual forging rapid occurs in a localised area, the heat maximum forging temperatures cooling of the workpiece must be will not dissipate into the workpiece must be used. - The Chromium avoided (e.g. indiscriminate use of water due to the low thermal conductivity. Equivalent may be determined sprays, chilling of the surface layers by - Overheating can therefore result by using the Schaeffler-DeLong contact with the die) as this” quenching” with associated formation of diagram as for predicting the crystal could cause incipient cracking within the undesirable crystal structures (delta structure of weld metal (Refer to the surface layers of the workpiece. FERRITIC STAINLESS STEELS These are plain chromium stainless TABLE 2: Forging of Martensitic Stainless Steel steels which have a low C content. They GRADE PREHEAT MAX FORGING MIN FORGING cannot be hardened/strengthened by (oC) Temp Range (oC) Temp (oC) heat treatment. The poor weldability 410 1100-1200 900 is an inhibiting factor which limits 416Se = 1150-1225 925 the application of the standard ferritic 420 +-775 1100-1200 900 stainless steels. 431 = 1150-1200 900 The main factor in forging these 440A,B,C +-775 1040-1150 925 steels is their susceptibility to excessive grain growth at the high temperatures employed for forging. This is due to their TABLE 3: Forging of Ferritic Stainless Steel single phase crystal structure. They must GRADE PREHEAT MAX FORGING MIN FORGING therefore be heated with care. (oC) Temp Range (oC) Temp (oC) Heating involves two stages: 405 +-850 1040-1110 900 • First to an intermediate temperature 430 +-850 1040-1110 820 of ± 850°C and then, once the 446 +-850 1040-1120 800 workpiece has uniformly attained 3CR12 +-850 1040-1100 800 this temperature, heated as quickly as possible to the forging great care to ensure that lower temperatures are not attained). the forging temperature, and not finished at higher temperatures because • And then immediately “tempering” subsequently subjected to sufficient deformation becomes increasingly more at 650°-680°C before finally allowing deformation during forging to fully difficult at lower temperatures. to cool freely in air to ambient refine the microstructure. • Soaking at the forging temperature temperature. The suggested forging temperature must not occur (i.e. to minimise the For complex forgings which have ranges of some of the ferritic grades extent of grain growth). large and/or sudden changes in of stainless steel are given in the table • The forging temperature must thickness of cross-section it may prove above. correspond to the amount of necessary to carry out a full annealing Cooling after forging should be by deformation which will take place process immediately after forging. cooling freely in still air. in order to optimise the refinement A further aspect related to cooling which will occur during forging. is that during the actual forging rapid AUSTENITIC STAINLESS STEELS - This is particularly important if cooling of the workpiece must be This classification of stainless steels two or more excursions to the forge avoided (e.g. indiscriminate use of water is extensively used to produce forged are necessary. Especially in the final sprays, chilling of the surface layer components due to their excellent excursion sufficient deformation to overcome this a modified cooling corrosion resistance and, additionally, must take place, and forging must sequence is required, viz air cool to the broad scope of their related finish at as low a temperature as ±250°C (taking note of the cooling effect) properties. possible. as this ”quenching” could cause incipient In general forging terms, the cracking within the surface layers of the austenitic stainless steels may be workpiece. considered as “friendly”. Take care to ensure that lower • Most of the grades may be forged temperatures are not attained, and then AUSTENITIC STAINLESS at higher temperatures without immediately “tempering” at 650°-680°C associated shortcomings. This before finally allowing to cool freely in STEEL IS CONSIDERED compensates for their greater high air to ambient temperature. temperature strengths and facilitates A further aspect related to cooling TO BE “FRIENDLY” their deformation during forging. is that during the actual forging rapid - However the compositions (e.g. cooling of the workpiece must be IN FORGING TERMS 309S, 310S) which give a higher avoided (e.g. indiscriminate use of water Chromium Equivalent tend to result sprays, chilling of the surface layers by in the formation of delta ferrite contact with the die) as this” quenching” at high temperatures. The upper could cause incipient cracking within If the coarse grain size is not forging temperature for such grades the surface layers of the workpiece and refined the material will exhibit inferior is accordingly limited. greater resistance to deformation at lower properties, e.g. in respect of toughness, The stabilised grades (321 and 347) temperatures may induce mechanical ductility and fatigue. may contain stringers/segregates of failure by tearing. • It is therefore advisable not to carbides/carbonitrides which can initiate The standard grades can, depending use ferritic stainless steels for the rupture during forging. on C content, begin to sensitise if production of forgings of such shape/ Grade 347 is less susceptible, and is the temperature falls below ±850°C. size that would result in any portion thus often the preferred/specified grade This is seldom a factor as forging is of the workpiece being heated to of stabilised steel for forging purposes. throughout the workpiece must TABLE 4: Forging of Austenitic Stainless Steel be avoided. - Heating time are GRADE MAX FORGING MIN FORGING ±10-20% less than those required Temp Range (oC) Temp (oC) for austenitic stainless steels of 304 and 304L 1150-1250 925 equivalent cross-section. 304N 1150-1250 975 • Forging should finish at as low a 303 and 303Se 1150-1250 925 temperature as possible, but must 309 and 309S 1150-1180 975 not be below 900°C = care must be 310 and 310S 1150-1180 975 exercised to prevent any section 316 and 316L 1150-1250 925 of the forging cooling at a higher 321 1150-1250 925 rate, or being chilled, to below this 347 1150-1225 925 temperature. • Initial forging should be effected without major reductions or change of shape. Once the material starts • The high forging temperatures are given above. to “flow” progressively more employed will induce a degree of • Preheating to an intermediate deformation/reduction can be grain growth. - Heating times must temperature of ±925°C is advisable accomplished. - The workpiece may compensate for the lower thermal for the heating of workpiece of be prone to failure by cracking/ conductivity of austenitic stainless cross- section thickness >175 mm. tearing during rapid deformation steels, but soaking for times longer Cooling after forging MUST be forging processes (e.g. single blow than that required to uniformly immediate (by quenching in water). CDF operation). It is beneficial if the attain the forging temperature • This is necessary for ALL grades, workpiece can be given some initial throughout the cross-section of including the “L” and the stabilized deformation before being set in the the workpiece must be avoided. grades. dies for the CDF operation. - The top temperature to which the • The indicative forging temperature workpiece is heated must correlate DUPLEX STAINLESS STEELS range is: to the amount of deformation/ This classification is being used to Max Forging Temp 1150°C reduction which is to be carried out an increasing degree to produce forged Min Forging Temp 925°C (i.e. to effect optimal refinement). components for applications which Preheating to an intermediate Forging should finish at as low a necessitate its higher resistance to pitting temperature of ±900°C may prove to be temperature as possible. and stress corrosion cracking in chloride of benefit in the heating of workpieces of • The higher alloyed grades (e.g. environments. Their higher strength large cross-section (>200mm). 309, 310) and grades alloyed with (cf austenitic stainless steel) is also an • Cooling after forging should be Nitrogen(N) have greater high advantage rapid, usually by cooling freely in temperature strengths. Therefore, Most of the duplex stainless steels still air. the finishing temperature is higher available are proprietary grades as because their greater resistance to produced by different manufacturers. PRECIPITATION HARDENING (PH) deformation at lower temperatures • Therefore details regarding the STAINLESS STEELS may induce mechanical failure by forging parameters should be This classification of stainless steels is tearing = the standard grades can, obtained from the manufactures as used in applications which demand the depending on C content, begin to specific requirements may apply. combination of high corrosion resistance sensitise if the temperature falls However, in general terms the and high strength, and which are often below ±850°C. This is seldom a following factors are pertinent. for a specific application of a critical factor as forging is finished at higher • These are dual phase alloys of a nature. temperatures because deformation mixed Ferrite/austenite crystal • Therefore the production of forgings becomes increasingly more difficult structure. may be considered to be of a at lower temperatures. • The maximum forging temperature “specialised” nature. Austenitic stainless steels can develop must be such as to induce a minimum Most of the precipitation hardening a greater resistance to deformation as coarsening of the ferrite fraction stainless steels are available as the amount of deformation increases, and, due to the high Chromium proprietary grades. especially as the temperature of the Equivalent, the formation of • They are complex materials which workpiece falls to the lower temperatures delta ferrite and sigma phase. can have specific forging parameters within the forging temperature range - The temperature to which the related to each grade, and the final (i.e. a “work-hardening” effect) = workpiece is heated must correlate properties are highly dependent on increased forging pressure or number of to the amount of deformation/ the thermo-mechanical procedures blows should therefore be expected and reduction which is to be carried out employed. allowed for in the final stages of a forging (i.e. to effect optimal refinement). Because of varying forging procedures sequence. - Soaking at the forging temperatures and parameters which apply, full details Forging temperature ranges for some for times longer than necessary should be obtained from the manufacture of the austenitic grades of stainless steel to attain uniform temperature of the specific grade which is to be forged. POST FORGING REQUIREMENTS rapidly cool by quenching in water. from high temperature exposure during Inspection and Testing Requirements forging and heat treatment. This scale The purchasing requirements DUPLEX STAINLESS STEELS impairs the corrosion resistance and which cover forged components will These steels are machined and used must be properly treated as the final set out any inspection and testing in the fully solution annealed condition operation before the forging is placed in required to validate their integrity and which is essential to develop their service. Often the forging is machined mechanical properties. Machining and optimal corrosion resistance. before entering service, so this process the development of the optimal aqueous • The annealing process requires is unnecessary. corrosion resistance must be borne in quenching in water from an • Scale may be removed by mechanical mind. annealing temperature which varies or chemical (pickling) methods, or a for the different grades. combination of both. HEAT TREATMENT • If mechanical methods are used, Heat treatment is required to produce PRECIPITATION HARDENING contamination of the surface of the required mechanical properties and STAINLESS STEELS the Stainless steel by Iron(Fe) or the optimal corrosion resistance for all These steels require to be solution steel particles MUST be avoided. types of stainless steel forgings. annealed for machining purposes. This Mechanical methods of scale treatment also takes all the constituent removal should be followed by a MARTENSITIC STAINLESS STEELS alloying elements uniformly into passivation Treatment. These are usually annealed (fully solution. • Pickling is the preferred method of softened) to render them amenable • The temperature used, the time scale removal. to machining. Different annealing at this temperature, and the Forgings which are to be used in high parameters apply to each grade. subsequent quenching rate will vary temperature application (>±600°C) do After machining the steel is then for the different grades. not need to be pickled and/or passivated. hardened and tempered to produce To develop the mechanical properties the desired mechanical properties further heat treatment is required. CONCLUSION of strength, hardness, ductility and • This is the precipitation hardening Although the tonnage of stainless toughness. Again, each grade requires heat treatment which is carried out steel forged is relatively small compared different quenching temperatures/ rates, at lower intermediate temperatures. to other wrought products, it nevertheless and tempering temperatures. - It is time-temperature related, constitutes a vital and indispensable The best aqueous corrosion resistance which varies for each grade sector of the industry. of the Martensitic Stainless steels is and depends on the mechanical The forging of stainless steel is attained in the hardened and tempered properties. essentially not difficult. But it is different, condition. Full details should be obtained for and often vastly so, from the forging of each steel from the manufacturers. plain carbon and low alloy steels. Further, FERRITIC STAINLESS STEELS Stainless steel cannot be considered as These steels are machined and used MACHINING a single material for forging purposes in the annealed condition. Different Stainless steels are not the easiest as the forging properties of the various annealing parameters will apply to each of materials to machine, and are classifications differ substantially. grade. therefore less tolerant of any shortfalls in The main factors of difference which AUSTENITIC STAINLESS STEELS machining technique. must be catered for are: These steels are machined and used • The heating of the stock for forging. in the fully solution annealed condition OPTIMAL CORROSION RESISTANCE • The higher strength at high which is essential to develop their In addition to the development of temperatures. optimal aqueous corrosion resistance. optimal aqueous corrosion resistance • The forging temperature ranges. • An equivalent annealing process by heat treatment as outlined above, the • Thorough refinement in the forging is used for all austenitic stainless passivity of the surface must be ensured. operation. steels, viz heat to 1050°-1060°C and Scaling of the surface will result • Cooling after forging.