High-Quality Tool Steel Hot Forming and Heat Treatment 2 CONTENTS

HOT FORMING 4

BASIC PRINCIPLES OF HEAT TREATMENT 6

TYPES OF HEAT TREATMENT 14

3 Hot Forming

FORGING For each steel the temperature range within which forging After forging, practically all hot-work steels must be cooled must be carried out is stated. slowly to avoid tension cracks.

The pieces to be forged must slowly be brought to the initial For high-alloyed and thus very transformation-resistant forging temperature. This has to be considered even more steels a special preliminary heat treatment may become the higher the steel is alloyed, as the heat conductivity necessary. For this, the pieces are to be caught at 500 to decreases normally with the alloying content. If heating is 550 °C coming from the forging operation and to be carried out too fast, internal cracks and other defects can charged into a furnace which is at the temperature equal occur. to the shortest pearlite transformation time (pearlite nose) in the isothermal TTT (time-temperature-transformation) Forging must be finished roughly near the minimum tem- diagram. The holding time is to be determined 3 or 4 times perature stated. The initial forging temperature should be as long as stated in the TTT diagram. coordinated with this. If the required minimum temperature is reached before finishing the forging process, re-heating must be carried out.

4 HOT FORMING HOT

WORKING ALLOWANCE Hot forming leads to a certain scaling and decarburization Concerning the suppliable types of design, the following can on the steel surface. Moreover, minor surface defects, such be distinguished: as pits, grooves and others, may remain. So as to avoid • Rolled or forged insufficient surface hardness of the tools or heat treatment • Rough-machined cracks, it is advisable to carry out chip machining prior to • Micro-finished hardening. The ordering dimensions for bars in rolled or forged as well For ordering, the relevant material allowances must there- as rough-machined condition may be taken from DIN EN fore be taken into consideration. The ordering dimension, ISO 4957. the dimension defined by the customer, is dependent on the finished dimension, i.e. the intended dimension of the It is acceptable for finished machined bars to deviate from tool after finish-machining as well as the type of design. customers ordering dimension according to DIN EN 10278.

5 Basic Principles of Heat Treatment

1600 Steels are iron-carbon alloys whose properties can be δ-Solid solution A Melt + δ-Solid solution B Fe C 1500 3 H (Cementite) influenced through changes of the chemical composi- δ- + -γ-Solid solution I Melt 1400 N tion (C-content and addition of alloying elements) and D 1300 Melt + γ-Solid solution Melt + Fe C through heat treatment. Thereby, a multitude of tool 1200 3 γ-Solid solution (Austenite) F steels is created which meet every requirement. To 1100 E C

1000 understand the different kinds of heat treatment, it is

γ-Solid solution + Fe1C Temperature in °C 900 G necessary to become familiar with the processes which γ- + α-Solid solution 800 occur in steels during heating and cooling. M O α-Solid solution K (Ferrite) 700 P S

α-Solid solution + Fe1C 600 The phase diagram iron-carbon (Figure 1) forms a 500 Q 1 2 3 4 5 6 L 7 first basis for heat treatment. It shows the structure Carbon content in weight percentage components and volumes under conditions of equilib- 0 10 20 30 40 50 60 70 80 90 100 Cementite in weight percentage rium. From the phase diagram it can be deduced that

Figure 1 austenite, and in hypereutectoid steels austenite and Phase diagram Iron-carbon cementite, are present at temperatures above the GSK line (see diagram). During very slow cooling, which again leads to conditions of equilibrium at room temperatures, a transformation of the austenite into other types of structure occurs.

6 Steels with C-contents of below 0.8 % precipitate ferrite in the TTT diagrams; they form a further basis of heat treat- from the austenite area during cooling, and the residual ment. BASIC PRINCIPLES OF HEAT TREATMENT HEAT OF PRINCIPLES BASIC austenite disintegrates below 723 °C into pearlite. At a carbon content of 0.8 % only pearlite is formed as a mixture In the isothermal TTT diagrams the time in the logarithmic of ferrite and cementite. In steels with C-contents of above scale is entered on the x-axis and the temperature on the 0.8 % pearlite and cementite are formed, whereby the sec- y-axis. ondary cementite is precipitated at the grain boundaries. The curves shown therein mark the transformation behavior of the supercooled austenite at isothermal heat treat- Through the addition of other alloying elements the trans- ment as previously explained. formation temperatures and the lines of equilibrium can be altered and the formation of carbide can be influenced.

The phase diagram does not give any information about the structure of steels cooled down fast from the austenite area. With higher cooling speed the transformation of the austenite does not occur according to the equilibrium ratios and leads to other kinds of structure – sorbite, troostite, bainite, martensite – which are not shown in the Fe-C- phase diagram. The transformation processes are described

7 In Figure 2 to 3 it is shown in what way these diagrams are Austenitizing temperature developed. Small samples heated to austenitizing temper-

ature – normally the hardening temperature – are quickly Cooling in the salt bath to transformation temperature submerged in a salt bath held at the desired transforma- Start of Finish of transformation transformation tion temperature according to Figure 2. They are kept in ~ Transformation Temperature in °C temperature the bath for increasing periods of time and then quenched Quenching in water in salt water. Following this, hardening measurement and structure examination are performed. In this, the time of

the start of the transformation B and its finish E are deter- Time in seconds

mined. This can also be carried out in a dilatometric way. Martensite Transformation structure

Such series of experiments are carried out for a range of Figure 2 Diagram of the determination of the transformation start time and temperatures. For example, Figure 3 shows the result for finish at isothermal heat treatment a steel with 0.55 % C and 1.04 % Mn. In Figure 3 all points B and, respectively, E are connected with each other. With 450 s (= 7½ mins.). For temperatures of 350 °C for example, this, the TTT diagram has already been outlined. transformation into bainite starts after a holding time of around 30 s. This is finished after 3,000 s (= 50 mins.). Its statement is further explained using the example of Below the Ms line martensite is formed. It forms sponta- the isothermal TTT diagram for the steel grade 1.2542 neously on reaching transformation time. Its amount – in (45WCrV7) as shown in Figure 4. Firstly, it turns out that 1st approximation – is dependent only on temperature and the austenite can be supercooled below Ac1 if it is cooled not on time. In Figure 4, for example, at 235 °C 50 % and at sufficiently fast. The supercooled state remains at first 100 °C 90 % martensite are preserved. Especially for hot- even during the isothermal treatment. Only while exceeding work steels at cooling to room temperature, i.e. at hard- the “rise time”, i.e. to the right of the “transformation start” ening, 100 % martensite can practically never be achieved. line, is a new structure component formed. In Figure 4, this There remains a certain amount of “retained austenite”, is initially ferrite at higher temperatures, e.g. at 700 °C, lat- whose transformation – frequently only in parts – can be er pearlite is formed. Transformation in this case ends after induced through a deep cooling treatment.

TTT DIAGRAMS FOR CONTINUOUS COOLING The continuous TTT diagrams are important resources for The diagrams state which transformations happen in a the practical heat treatment, especially hardening. steel depending on the cooling speed and which hardness comes about after achieving ambient temperature (circled These diagrams are to be read in the direction of the numbers on the lower curve end). Also the constituents marked-in cooling curves which (in the case of round of the microstructure including the retained austenite are bars) are to be aligned, depending on the diameter and on stated in the diagrams. the quenching medium applied, to the individual section areas, e.g. rim, mid-radius and center. The fast coolings are marked by so-called parameters, the slower ones through direct statement of the cooling speed. The parameters come about through division of the cooling time from 800 to 500 °C by 100 (e.g. parameter 0.15: the curve for which cooling from 800 to 500 °C needs 15 s).

8 Austenitizing temperature Figure 3 Development of an isothermal TTT diagram for a steel with Cooling curve 800 0.55 % C and 1.04 % Mn

B E 700 B E

B E 600 B E

B E 500 B E

B E 400 Temperature in °C B E

B E 300

200

100

0 0.1 1 101 102 103 104 105 Time in seconds

Austenitizing temperature: 900°C 1200 Figure 4 1.2542 (RTW 2 H) 1100 Isothermal TTT diagram for the steel 1.2542 (45WCrV7) 1000

900 Δ Ac3 (0.4°/min) Start of pearlite formation 800 Δ Ac1 (0.4°/min) Start of ferrite formation

Finish of transformation 700 70 30 23 96 32 34 35 Pearlite phase 600 Austenite + carbide 36 37 Start of transformation 500 Temperature in °C 400 M Start of transformation S 44 40 300 47

54 Bainite TREATMENT HEAT OF PRINCIPLES BASIC 50% 200 Martensite phase 100 90%

61 0 1 10 102 103 104 105 106 Seconds 1 2 4 8 15 50 60 1 2 3 5 7 10 = Hardness values in HRB or, respectively, HRC Minutes Days 70 = Structure percentage 1 2 4 8 16 24 Time Hours

Figures 5–7 make clear the typical differences which occur sion proceeds so slowly that even at 0.2 °C/min. only 20 % in the TTT diagrams of steels of different hardenability. For of pearlite is generated. the water hardening steel 1.1625 (C80W2) in Figure 5, the It should be emphasized that the diagrams displayed in- complete conversion in the pearlite phase occurs already clude a certain scatter range due to a case of analysis and in fast cooling (parameter 0.03). For the oil hardening steel is only valid for the stated austenitizing conditions and a 1.2542, cooling must amount to only max. 10 °C/min. normal state of pre-treatment. Higher austenitizing tem- according to Figure 6. For the air hardening steel peratures push conversion, especially the pearlite phase, 1.2767/RABW (45NiCrMo16), however, the pearlite conver- back, they therefore improve hardenability.

9 Austenitizing temperature: 800°C 1200 Figure 5 1.1625 (RB 8) 1100 Continuous TTT diagram for water hardening steel. 1.1625 (C80W2) 1000 0.85 % C, 0.15 % Si, 0.20 % Mn 900

800 Ac3 (0.4°/min)

700 100 100 100 100 Ac1 (0.4°/min) 100 100 100 100 Pearlite phase 600 Austenite 100 + carbide 100 0.006 0.03 0.04 0.08 0.15 0.28 0.50 1.2 2.6 20°/min 5°/min 500 2 4 98

Temperature in °C 0.009 0.02 400 0.01

300

MS 200 Martensite phase 100

876 0 870 865 845 440 378 358 326 314 300 284 273 256 233 212 0,1 1 10 102 103 104 105 Seconds = Hardness values in HV 1 2 4 8 15 50 60 2 – 100 = Microstructure constituents in % Minutes 0.006 – 2.6 = Cooling parameters 1 2 4 8 16 24 Time Hours

Austenitizing temperature: 900°C 1200 Figure 6 1.2542 (RTW 2 H) 1100 Continuous TTT diagram for oil hardening steel. 1.2542 (45WCrV7) 1000 0.45 % C, 0.95 % Si, 1.10 % Cr, 0.17 % V, 1.90 % W 900 Ac3 (0.4°/min)

800 Ac1 (0.4°/min) 43 Pearlite phase Ferrite 40 700 40 57 35 60 30 60 Austenite + carbide 1 10 600 Pearlite 20°/min 500 0.14 0.30 0.50 1.3 2.6 7.0 10°/min 5°/min 1.25°/min Temperature in °C 400 Bainite MS 3 95 98 300 70 68 54 Martensite phase 200

100

730 699 690 579 498 454 415 395 347 251 228 0 1 10 102 103 104 105 106 Seconds = Hardness values in HV 1 2 4 8 15 50 60 1 2 3 5 7 10 1 – 98 = Microstructure constituents in % Minutes Days 0.14 – 7.0 = Cooling parameters 1 2 4 8 16 24 Time Hours

Austenitizing temperature: 840°C 1200 Figure 7 1.2767 (RABW) 1100 Continuous TTT diagram for air hardening steel. 1.2767/RABW (45NiCrMo16) 1000 0.45 % C, 1.35 % Cr, 0.23 % Mo, 3.9 % Ni 900

800 Ac1e (0.4°/min)

700 Ac1b (0.4°/min)

600 Pearlite Austenite 20 500 + traces of carbide 0.14 0.4 1.1 2.1 5.6 20°/min 10°/min 5°/min 2.5°/min 1.25°/min 0.4°/min 0.2°/min Temperature in °C 400

300 M Bainite S 1 50 98 78 200 Martensite phase 100

726 623 613 613 613 613 613 603 603 584 532 440 435 0 1 10 102 103 104 105 106 Seconds = Hardness values in HV 1 2 4 8 15 30 60 1 2 3 5 7 10 1 – 98 = Microstructure constituents in % Minutes Days 0.14 – 5.6 = Cooling parameters 1 2 4 8 16 24 Time Hours

10 Austenitizing temperature: 850°C 1200 Figure 8 1.2714 (RGS 4) 1100 Isothermal TTT diagram and isothermal transformation in the pearlite phase types of heat treatment 1000 isothermal transformation in the bainite phase 1.2714/RGS 4 (55NiCrMoV7) Hot-bath hardening 900

Ac (0.4°/min) 800 1e

700 Start of transformation Ac1b (0.4°/min) 27 26 End of transformation Austenite + carbide 30 31 600 Pearlite phase 34

500 Start of transformation Temperature in °C 400 End of transformation

300 51 47 Bainite MS 55 200 50% Martensite phase 100 90% 63 0 1 10 102 103 104 105 106 Seconds = Hardening values in HRC 1 2 4 8 15 30 60 1 2 3 5 7 10 Minutes Days 1 2 4 8 16 24 Time Hours

PRACTICAL APPLICATION OF THE TTT DIAGRAMS FOR CONTINUOUS COOLING So as to be able to transfer the statements of the TTT table. It states for water, oil and air hardening the cooling diagrams to the hardening of tool steels one must know the parameters λ1 or, respectively, the cooling speeds ν in the cooling speed of water, oil and air hardening. For round bars core depending on the diameter. this can be stated relatively exactly. If one at first concen- trates on the core zone it is possible to use the following In order to determine the cooling speed or the cooling pa- Ø (mm) for cooling in rameter on various section areas, the following Figures 9 to λ respectively ν1 water2 oil2 air 13 are to be consulted. 0.01 10 They are valid for water, oil, and air cooling and round bars 0.03 18 of 50 to 800 mm diameters. 0.06 25 10 The broken line refers to the core of the round rods.

0.10 35 18 TREATMENT HEAT OF PRINCIPLES BASIC 0.20 50 30 For the cooling parameter or the cooling speed determined 0.35 70 45 in this way, the structure and the hardness to be reached 0.50 85 60 after conversion can be determined in the TTT diagram. 0.70 105 70 Some examples can clarify the possibilities of application: 1.00 130 90 10 2.00 180 130 18 3.00 230 170 25 Example 1: Which surface hardness can be reached with 5.00 300 220 40 the oil hardening steel 1.2542 on a cylindrical work piece 7.00 360 280 55 with a diameter of 100 mm? 9.00 20°/min. 420 320 65 18.00 10°/min. 600 500 130 According to Figure 11 the parameter for a diameter of 36.00 5°/min. 900 750 220 100 mm and a distance from the edge of 5 mm amounts to 72.00 2.5°/min. 1300 1150 400 0.29. In the continuous TTT diagram for 1.2542 (Figure 6) 144.00 1.25°/min. 1900 1700 700 a cooling curve with a parameter of 0.30 is found. For this 225.00 0.80°/min. 2400 2200 1000 parameter a hardness of 690 HV and a structure with 97 % 450.00 0.40°/min. 3500 3300 1900 martensite and 3 % bainite is achieved.

1The cooling parameters or, respectively, speeds are attached to the cooling curves on the 500 °C isotherms in the TTT diagram. 2Workpiece moderately moved. 11 Example 2: A cylindrical die of 400 mm diameter from steel 1.2767/RABW is air hardened. Which hardness and which structure are to be expected at the edge and in the core? 4 3 200 2 ) -2 150 According to Figure 13, edge and core cool down with 1

nearly the same speed of 2.5 °C/min for a diameter of 400 λ 0.8 100 0.6 mm. According to the TTT diagram for continuous cooling 0.5 80 Cooling curve 0.4 for the core of (Figure 7) this cooling generates in the core 1 % bainite, 94 % 0.3 round bars

martensite and 5 % residual austenite for a hardness of 584 Cooling parameter 0.2 50 HV. For the edge it can be deduced that 95 % martensite and 0.1 (Cooling from 800 °C to 500 in sec x 10 5 % residual austenite for a hardness of about 600 HV can 0.08 0.06 be achieved. 0.05 0.04 0 10 20 30 40 50 60 70 80 90 100 min Distance from the surface in mm All these data are valid for the hardened condition before Figure 9 tempering. Water quenching

100 10 80 8 60 6 50 5 ) ) -2 -2 40 5°/min 4 200 30 800 3 150 700 20 10°/min 2 λ 600 500 100 10 20°/min 1 400 8 Cooling curve 0.8 80 Cooling curve for the core of 6 for the core of 0.6 300 round bars Cooling parameter λ Cooling parameter 5 round bars 0.5 4 0.4 50 3 0.3 200 (Cooling from 800 °C to 500 in sec x 10 (Cooling from 800 °C to 500 in sec x 10 2 0.2

1 0.1 0 40 80 120 160 200 240 280 320 360 400 0 10 20 30 40 50 60 70 80 90 100 Distance from the surface in mm Distance from the surface in mm Figure 10 Figure 11 Water quenching Oil quenching

100 400 80 2.5°/min 1000 300 60 50 200 1°/min 800

) 800 ) -2 40 5°/min 700 -2 1.25°/min 700 600 30 600 100 500 20 10°/min 500 80 2.5°/min 400 λ λ 400 60 300 50 Cooling curve 10 20°/min 40 5°/min for the core of 300 200 8 Cooling curve 30 round bars 6 for the core of

Cooling parameter round bars Cooling parameter 5 20 10°/min 200 4 100 3 10 20°/min (Cooling from 800 °C to 500 in sec x 10 2 (Cooling from 800 °C to 500 in sec x 10 8 6 5 1 4 0 40 80 120 160 200 240 280 320 360 400 0 40 80 120 160 200 240 280 320 360 400 Distance from the surface in mm Distance from the surface in mm Figure 12 Figure 13 Oil quenching Air quenching

12 BASIC PRINCIPLES OF HEAT TREATMENT HEAT OF PRINCIPLES BASIC

13 Types of Heat Treatment

Already the design of the tools by the design engineer corrosion and other residues contaminate could have a should take the heat treatment into consideration. Large harmful effect on the heat treatment process. differences in section, especially with sharp transitions and edges are to be avoided as they lead to locally high stress- In this brochure, treatment instructions for every type of es. steel are provided for the hot forming and heat treatment. These instructions are guidelines and do not exclude special A clean surface of the tools is very important. Scratches measures in individual cases. create grooving effects and cause crack formation. Scale,

NORMALIZING When a tool steel has been hot-formed one more time It is about 30 °C above the hardening temperature. They in the course of further processing, normalizing before are held at this level for about ½ hour and subsequently soft-annealing is expedient. Normalizing equalizes differ- cooled in air. ences in structure and/or refines a coarse grain. Through this, an initial structure, as uniform as possible, is achieved A complete transformation in the pearlite stage is the for the subsequent heat treatment. For this, the tools are prerequisite for the desired grain refinement. Only this slowly and thoroughly heated to normalizing temperature. transformation leads to the desired grain refinement; not,

14 however, the transformations in the bainite phase and the isothermal tempering transformation in the pearlite phase martensite phase. through the application of the isothermal TTT diagram. Cooling down from the normalizing temperature into the It can be determined from the continuous TTT diagram, range of the pearlite formation can be carried out in the whether a complete or nearly complete pearlitic transfor- furnace or through recharging. It is expedient to select as mation through normalizing can be reached for the given di- austenitizing temperature a temperature which lies be- mensions for a certain steel. According to the TTT diagram tween 20° and 40 °C above Ac1e (Ac1e is defined in the TTT for continuous cooling from the austenite area, higher-al- diagrams). As this is lower than the austenitizing temper- loyed and thus transformation-resistant steels are to be atures referred to in the TTT diagrams, the transformation cooled so slowly that the steel transforms completely in takes place faster than stated in the TTT diagrams. This the pearlite phase. The same effect can be reached with an means that there is a welcome safety factor.

SOFT ANNEALING

The soft-annealed condition is the most appropriate for annealing on the part of the customer is taken into account TREATMENT HEAT OF TYPES machining and chipless processing of the steels. Moreover, only if the steel is reforged or a second hardening is car- it is favorable for subsequent hardening. Tool steels are ried out by the customer. During soft annealing the steel is thus delivered in general in soft-annealed condition. Soft slowly heated to the required temperature – in the vicinity

15 of the transformation temperature Ac1b – and held on this The soft-annealing temperatures for a lot of steels are level for two to four hours, then slowly – 10 to 20 °C/h stated in our brochures. Soft-annealed steel has a charac- – cooled down to around 500 °C in the furnace and sub- teristic structure, so-called granular or globular cementite. sequently cooled in air. Suitable measures are to be taken against scaling and decarburization.

STRESS RELIEVING Components with complicated forms or widely differing particularly critical cases, to air-cool it and to subsequent- sections will always warp more or less strongly during ly soft-anneal it so as to enable finish-machining. In the hardening. So as to prevent warping through treatment course of this “pre-tempering”, the complicated component stresses during later heat treatment, a stress relieving of will deform under stress reduction. Through finish-machin- the components in rough-machined condition is recom- ing, it can be adjusted to the correct dimensions. Self- mended. For this, temperature should be 600 to 650 °C. evidently, sufficient material allowances are necessary. For Holding time: Depending on wall thickness one to several final tempering, the usual grinding tolerances are generally hours, with subsequent furnace cooling. sufficient.

The so-called stress relieving, however, is often not suf- Through frequent and regular stress relieving, the ser- ficient to reduce possible internal stresses, which may be vice-life of hot-forming tools in operation can be considera- present in the component for other reasons. Thus, it is bly extended. This heat treatment is expediently carried out expedient to austenitize the rough-machined component in at about 30 to 50 °C below the tempering temperature.

HARDENING When the tools arrive for hardening they are mostly fin- of the tools with the least possible risk of rejects, the heat ished or nearly finish-machined save for a small grinding treatment must be carried out with the highest possible allowance. So as to be able to reach the desired properties care.

SURFACE PROTECTION During heat treatment, chemical processes occur with The highest degree of surface protection of workpieces is heightened speed at high temperatures. Thus, during heat reached in vacuum furnaces.

treatment the problem arises of keeping the thermochem- Carbon neutral ical influence of the heating media on the steel surface of a Decrease Increase Tendency

workpiece neutral. Even if the surface of the workpiece has Chamber furnaces Unstable with packaging remained blank during hardening, the surface area may be chemically changed and damaged through partial decarbur- CO-containing Carburization inert gases ization, carburization, oxygen and nitrogen intake (Figure Inert gas Decarburization 14). CO-free

Low Salt baths decarburization The various heating media such as firm packaging media, inert Oxidation hardening salts and inert gases display a more or less neu- neutral Vacuum furnaces tral behavior and are thus manageable to varying degrees X-axis = frequency (Figure 14). Many years of experience and continuous Figure 14 control of heating media will yield good, steady results. Influence on the surface in various hardening processes

16 HEATING TO HARDENING TEMPERATURE The higher the alloying elements in a steel, the lower its chamber, bogie hearth or similar furnaces in packaging, heat conductivity. This is why tool steels, which are in with inert gas or in vacuum, the prescribed preheating tem- general highly alloyed, must be heated carefully. If hard- peratures are also to be set as steps. The workpieces are to ening is not carried out from heat treatment facilities with be held on these temperatures up to full soaking. controlled heating speed, the tools are to be preheated in single or multiple steps. We state the most favorable pre- The tool has to be fully heated up to the prescribed hard- heating temperatures for the different tool steels. ening temperature after preheating. The hardening furnace must be controlled in such a way that the thinner sections of a component, which inevitably rush ahead in the heat- a = hardening 180 a section ing, are not over-heated or excessively held. Holding time,

160 High-alloyed steels which starts with the achievement of the temperature of (e.g. 12 % 140 Cr-steels) the complete component, is dependent on the steel grade 120 a and the chosen temperature within the prescribed range. a 100 In the case of simply-shaped tools, the upper hardening

80 Unalloyed or Duration in minutes low-alloyed steels temperature limit can be selected most of the time, yet in 60 the case of tools with strongly varying sections the lower 40 hardening temperature range is to be preferred. Therefore, 20 holding times in the former case are shorter than in the 0 0 20 60 100 140 180 220 260 latter. Unalloyed and low-alloyed steels must be held on Wall thickness in mm the hardening temperature for significantly shorter peri- Figure 15 Guiding values for the holding time after reaching the hardening ods due to the risk of overheating (grain coarsening) than temperature on the surface, or the immersion time in the salt bath respectively, in dependence of the wall thickness. high-alloyed, Cr-, Mo-, V-, W-carbide containing hot-work steels. For these, the solution of the carbides, important for For salt facilities it is recommended to preheat the compo- the achievement of optimal tempering resistance, needs nents in a furnace at 300 to 400 °C before charging them significantly more time. into a salt bath with the prescribed preheating temperature. Preheating may however be omitted in the case of small, Figure 15 serves as an indication for the determination of simply shaped tools, if the prescribed hardening tempera- the holding time after reaching the hardening temperature ture in the salt bath does not exceed 850 °C. If heating is on the surface or the immersion time in the salt bath. not carried out in salt baths but in furnaces such as retort,

QUENCHING Quenching must be performed with the prescribed harden- components are to be removed from the quenching bath ing agent. If hardening in water is prescribed, its temper- and immediately tempered. In the case of compressed ature should be at 20 to 40 °C. For the internal hardening or blast air quenching, perfect dryness of the air must be of bore holes, suitable sputtering equipment must be ensured. installed. Hardening oils are to have higher temperatures

according to the prescriptions of the manufacturer. As soon TREATMENT HEAT OF TYPES as the temperature has decreased to 60 to 100 °C, the

17 HOT-BATH HARDENING Hot-bath hardening offers particular advantages. For this, The workpieces are held in these baths at least to full quenching is carried out in an intermediate bath which is thermal equilibrium and subsequently cooled down to room held at a temperature in the transformation-resistant range temperature. of the isothermal TTT diagram. For example, for the steel grade 1.2379 (X153CrMoV12) the temperature of 450 to Hot-bath hardening, which can be carried out not only after 500 °C is considered favorable, in which the supercooled heating in a hardening salt bath but also from other fur- austenite is stable. For higher-alloyed steels such as high- naces, offers the advantage of reduced dimension changes, speed and hot-work steels a salt melt with a melting point weaker warping, the smallest risk of cracks and a clean of roughly 430 °C at a temperature of 500 to 550 °C is surface. used. For lower-alloyed oil hardenable steels a salt melt with a melting point of 140 °C is used at a temperature between 160 to 200 °C.

TEMPERING Tempering means holding the hardened steel at tempera- that the influence of temperature is much stronger than

tures below Ac1, mostly with subsequent air cooling. that of time. Multiple tempering increases toughness. Tempering causes a reduction of stress and a gain in For higher-alloyed steels and tempering temperatures of toughness through the transformation of the “tetragonal” over 500 °C, a second tempering at about 30 °C below martensite in the less crack-prone “cubic” form. Therefore, the temperature of the first tempering is required. This is tempering must in all cases be carried out immediately necessary due to the presence of retained austenite after after hardening. hardening, which, during cooling after the first tempering, transforms into the tetragonal martensite. This is convert- Moreover, through tempering a microstructure stable ed into the less brittle cubic martensite during the second against heat affection is achieved, which is particularly tempering. important for hot-forming tools. The precondition for the microstructure stability is that the tempering temperature Immediately after hardening the tools must best be selected is at least 30 to 50 °C higher than the working tempered in a convection air furnace, in a fluid bath (oil) or temperature of the tool. For this, however, the interchange- in salt baths. The temperatures are to be taken from the ability of temperature and time, which exists to a certain tempering diagrams of the individual steel grades. The extent, must be taken into consideration. tempering time is dependent on the material thickness. The components are to be slowly heated to tempering temper- For example, a one-hour tempering at 600 °C has some- ature and are to be held for at least one hour per 25 mm thing like the same effect as a tempering treatment at thickness at this temperature. 500 °C over a period of 400 hours. This example shows

18 19 CASE HARDENING Case hardening means a hardening after prior carburiz- lie at 870 to 930 °C, however also higher temperatures are ing, where appropriate with simultaneous nitriding of the occasionally applied. surface. Case hardened tools are preferably employed as structural steels for plastics processing, i.e. where high As a rule, tool steels are case hardened according to 2 core toughness and a wear resistant surface are required processes: together. 1. Direct hardening consists of a hardening of the carbur- The carburizing consists of a carbon concentration through ized workpiece at the end of the carburizing process,

holding at a temperature above the conversion points Ac1 whereby the hardening temperature may lie lower than

or Ac3 in carbon-emitting media. Depending on the type of the carburizing temperature; however it must still be

medium employed this is referred to as gas, bath or pow- above Ar3 of the case. der carburizing. The carburizing depth is determined by the 2. In single hardening the work piece is cooled after car-

duration of the carburizing and the activity of the carburiz- burizing to below Ar1 and subsequently quenched from ing medium. In general, the case hardening temperatures the hardening temperature of the case.

THERMAL CYCLE DIAGRAM The heat treatment processes described may be easily • Low-alloyed cold-work steels with hardening tempera- shown through thermal cycle diagrams for individual steel tures of below 900 °C. This group includes most of the groups. cold-work steels, such as 1.2842 (90MnCrV8), 1.2067 (102Cr6), 1.2419 (105WCr6), 1.2767/RABW (45NiCr- 1. Hot-Work Steels Mo16) and so on (Figure 19). With respect to heat treatment hot-work steels can be subdivided into two groups: 3. Unalloyed Tool Steels (Figure 20) • Hot-work steels with a hardening temperature of above Unalloyed steels have low effective hardening depth; they 950 °C (high-alloyed hot-work steels, Figure 16). are water-hardening steels. The quenching effect of water • Hot-work steels with a hardening temperature below and thus the hardness of the tool can be increased through 950 °C; this includes most of the die steels (Figure 17). the addition of salt (10–12 % salt solution). Oil-quenching is considered only for few grades and in this case only for Double tempering is required for hot-work steels with a very small thicknesses. hardening temperature of over 950 °C (high alloyed steels), for such with a hardening temperature below 950 °C (die 4. Case-Hardened Steels steels) it is expedient. The principle of multiple tempering Case hardening has already been described before. increases the toughness in these cases, too. The two thermal sequence diagrams for • direct hardening (Figure 21) 2. Alloyed Cold-Work Steels • single hardening (Figure 22) With respect to heat treatment cold-work steels just like will demonstrate the two processes. hot-work steels can be subdivided into two groups: • High-alloyed cold-work steels with hardening temperature of above 900 °C. This includes for example the high-chromium-alloyed steels of the type 2 % C–12 % Cr, as well as steels with a Cr-content of above 5 %, such as 1.2362 (X63CrMoV5-1) or 1.2363 (X100CrMoV5) (Figure 18).

20 Stress relieving Preheating – Austenitizing – Hardening Tempering

Hardening temperature Finish Rough machining 2nd preheating stage 1 min/mm machining Air/oil Slow ~ 900°C furnace cooling Equalizing Temperature 1st preheating stage 1st tempering temperature 1 h/25 mm 2nd tempering 600 – 650°C 1/2 min/mm 1 h/100 mm 1 h/25 mm ~ 650°C Hot bath 500 – 550°C Warming-up stage Air Air 1/2 min/mm ~ 400°C

Time

Figure 16 Thermal sequence for the heat treatment of hot-work steels with a hardening temperature of above 950 °C

Stress relieving Preheating – Austenitizing – Hardening Tempering

Finish Rough machining Hardening temperature machining Slow Equalizing furnace cooling Temperature Preheating stage temperature 1/2 min/mm 1 h/100 mm 600 – 650°C ~ 650°C Oil/air/ water Air Warming-up stage Tempering 1/2 min/mm 1 h/25 mm ~ 400°C

Time

Figure 17 Thermal sequence for the heat treatment of hot-work steels with a hardening temperature below 950 °C

Stress relieving Preheating – Austenitizing – Hardening Tempering

Hardening temperature Finish Rough machining 2nd preheating stage 1 min/mm machining Slow ~ 850°C Equalizing furnace cooling temperature Temperature 1st preheating stage 1 h/100 mm 600 – 650°C 1/2 min/mm ~ 650°C Hot bath Air/oil 500 – 600°C Air Warming-up stage Tempering 1/2 min/mm 1 h/25 mm ~ 400°C TYPES OF HEAT TREATMENT HEAT OF TYPES

Time

Figure 18 Thermal sequence for the heat treatment of high-alloyed cold-work steels with hardening temperatures of about 900 °C

21 Stress relieving Preheating – Austenitizing – Hardening Tempering

Hardening temperature Finish Rough machining 2nd preheating stage 1 min/mm machining Slow ~ 850°C Equalizing furnace cooling temperature Temperature 1st preheating stage 1 h/100 mm 600 – 650°C 1/2 min/mm ~ 650°C Hot bath Air/oil 450 – 500°C Air Warming-up stage Tempering 1/2 min/mm 1 h/25 mm ~ 400°C Hot bath ~ 200°C

Time

Figure 19 Thermal sequence for the heat treatment of low-alloyed cold-work steels with hardening temperatures of up to 900 °C

Stress relieving Preheating – Austenitizing – Hardening Tempering

Finish Rough machining Hardening temperature machining Slow Equalizing furnace cooling temperature Temperature Preheating stage 1 h/100 mm 600 – 650°C 1/2 min/mm ~ 650°C Air/oil/ Air water Warming-up stage Tempering 1/2 min/mm 1 h/25 mm ~ 400°C Hot bath ~ 200°C

Time

Figure 20 Thermal sequence for the heat treatment of unalloyed tool steels

Stress relieving Heating – Carburizing – Hardening Tempering

Finish Rough machining Carburizing temperature machining Slow Equalizing furnace cooling Temperature temperature Decreasing 1 h/100 mm 600 – 650°C and equalizing temperature Oil/water Air

1st Tempering 1 h/25 mm

Time

Figure 21 Thermal sequence for direct hardening

22 Stress relieving Heating – Carburizing – Cooling – Preheating – Hardening Tempering

Finish Rough machining Carburizing temperature machining Hardening temperature Slow furnace cooling Temperature Preheating stage 600 – 650°C Air cooling 1/2 min/mm ~ 650°C Oil/water Air

1st Tempering 1 h/25 mm Hot bath ~ 200°C

Time

Figure 22 Thermal sequence for single hardening

EXAMPLES FOR APPLICATION The opposition between toughness and wear resistance hard metals and steam tempering. With surface treatment, properties forces a reasonable compromise in the material not only an increase of the wear resistance is achieved selection and the determination of hardness. In many cas- through partially considerable hardness increases. At the es, the resulting wear resistance of the tool is not sufficient same time the sliding properties are improved and corro- for cost-efficient work life. Through surface treatments on sion resistance is increased. the finish-machined tool decisive improvements can however be achieved. From the multitude of possible processes These surface treatment processes are extensively de- only 4 shall be mentioned here, which have already met scribed in literature. In the following, especially their effects with a broader application or have recently gained greater on the surface properties of the steels will therefore be importance: nitriding, hard chromium plating, coating with shown.

NITRIDING One of the most frequent processes for tools today is 1.2080 (X210Cr12) are not suited for nitriding, the remain- nitriding. Depending on the process – gas or salt bath ni- ing steels can be treated. triding, ionitriding – treatment temperatures of around 450 70 to 580 °C are applied. Therefore, only steels with suffi- 66 cient tempering resistance are suitable for such treatment, 62 preferably those with a distinctive secondary hardness 58 maximum. On the one hand, the material must be tempered 54 sufficiently highly before nitriding so that during the final 50 treatment no dimensional changes caused by structural 46 Rockwell hardness HRC Material changes occur, which would make post-treatment of the number Hardening 42 Saarstahl temperature °C 1.2080 950 tools necessary. On the other hand, the base material must 38 1.2344 1050 1.2369 1070 1.2379 1080 retain a sufficiently high hardness so that a sufficient sup- 34 1.2842 810 1.3343 1220 port effect for the superhard surface layer is guaranteed. 30 TREATMENT HEAT OF TYPES 0 100 200 300 400 500 600 700 800 Tempering temperature in °C (duration 1h, air cooling) Mean values on samples 25 x 50 mm In Figure 23 the tempering diagrams of some tool steels Figure 23 are contrasted. Of these materials the steels 1.2842 and Tempering behavior of tool steels

23 Due to the advantage of short treatment times, often salt (diffusion zone) up to around 150 μm are reached. In order bath nitriding according to the Tenifer process is applied for to prevent flaking on the cutting edge of cutting tools, the materials. Depending on material and treatment time, sur- meeting of two nitrided surfaces should be avoided through face hardnesses of 1,000 to 1,300 HV and nitriding depths regrinding of one surface.

HARD CHROMIUM PLATING For hard chromium plating the surface hardnesses are in Adhesion has been improved against the traditional pro- the range of 1,000 to 1,200 HV. The treatment is carried cesses. As a rule, chromium thicknesses of 5–20 μm out at around 50 °C so that the basic structure is not af- thickness are generated. Tools chromium plated accord- fected. Adhesion is not always sufficient as a diffusion zone ing to this process in some instances reach considerably with the base material will not be created. A more sophis- increased efficiency. ticated method exists under the name Duralloy, with which hardnesses of 1,200 to 1,400 HV can be set.

COATING WITH HARD MATERIALS Very high hardnesses of around 2,000 to 4,500 HV can be and collaborators is quoted (Figure 24). For punching holes set if hard materials such as TiC, TiN or WC are emitted into plates of various thicknesses applying stamps of a 12 % from the gas phase to the tool surface. The two best- Cr-steel the influence of the TiC layer coated according to known possibilities are the CVD and the PVD processes. the CVD process was examined. The diagram shows that through coating the number of punches performed until In the chemical vapor deposition process (CVD) the coating regrinding is increased between 4 and 20 times depending material is generated through the chemical reaction of gas- on the thickness of the plates. eous components. The treatment temperatures lie around Figure 24 1,000 °C so that for tool steels only those with hardening 100 With TiC Tempering behavior of 50 temperatures within or above this range can be coated. Due 3 tool steels Without TiC to the unavoidable changes in dimension and form through 20

conversion and temperature stresses, only geometrically 10

simple tools can be treated as the low coating thickness 5

(around 5 μm) does not permit finishing. 2

In the physical vapor deposition process (PVD) the metal- 0.5

Number of hole punches until re-grinding in 10 Cutting material: 0.12 % C; 0.35 % Mn lic hard material component is present in solid form. It is Stamp: 1.5 % C; 12 % Cr (corresponds 1.2201) brought into the steam phase through physical processes 0.1 0.5 1 2 3 5 10 (evaporating, sputtering, ion-plating) where it deposits on Thickness of cutting material in mm the tool surface after bonding with the reactant. Against the CVD process the PVD process has the advantage that Hard material layers can also be deposited through a kind the coating temperatures (around 350 to 550 °C) lie within of spark discharge hardening whereby the surface is heated the range of the tempering temperatures of high-alloyed only to a depth of a few μm. In this process TiC or WC is tool steels. The tools can therefore be coated after the heat deposited through spark discharge between the tool and a treatment is finished. vibrating electrode. Layer thicknesses up to around 80 μm The increase in efficiency through coating with hard materi- are achievable; due to the high surface roughness regrind- als is considerable. As an example a research result by Sato ing must be performed.

24 STEAM TEMPERING Steam tempering is a surface treatment process which is The treatment is carried out in convection furnaces into applied nearly exclusively for high-speed tool steel. For which dry steam is introduced. The temperatures must this, an extremely thin oxide film of some thousandths mm lie below the latest tempering temperature, i.e. they are thickness is deposited on the tools. slightly above 500 °C. The process is especially suitable for small tools such as This oxide film reduces the tendency for adhesion and spiral drills and screw taps, broaches, reamers, counter- cold-weldability of the tools as well as their abrasive wear. sinks and such.

GRINDING The high operational speeds which are usual during grinding Hardened tools from high-speed steel as well as such of on modern grinding machines significantly increase the risk high-quality cutting steel, also case-hardened tools, for of burning the tools in case of too high forward feeds. which in most cases a clean grinding is necessary, must be ground with abundant water cooling. For cooling no soap- In the case of hard tool steel and high-speed steel, it is like coolants should be used if possible. Yet, the use of a mostly the dreaded grinding cracks created due to improper grinding emulsion is recommended. grinding operation, especially due to too hard disks. In the case of case-hardened components, the case may splinter One can immediately realize on the ground surface wheth- off due to unsuitable grinding. er the grinding was carried out properly or improperly. If the ground surface displays a yellowish to brownish color, For the selection of the disks, the following principle ap- grinding was carried out with an excessively deep chip, i.e. plies: Use hard disks for soft steel, use soft disks for hard with too much heat generation. If hardened tools show steel. such surfaces, they are tempered through grinding, i.e. they have become soft, and they mostly contain grinding cracks. For rough grinding use coarse-grained, for fine grinding use This can be recognized through slight etching with weak fine-grained discs. Too hard and too fine discs heat up the acids. steel too much and cause grinding cracks. When in doubt, the selection of the grinding discs should be determined by the producer.

ELECTRICAL DISCHARGE TEXTURING During electrical discharge machining the material on the bing should by all means be removed through one surface of the tool becomes fusible. As a consequence there or more coating erosion steps with low performance. remain from the outside to the inside a solidification area Moreover, a final stress relieving of the tool of around with cast structure, a hardness increase and a high-tem- 30 to 50 °C below tempering temperature is recommended pered layer. Below exists the unaffected base material. as also after coating erosion an affection of the surface The so-called white zone almost always contains tension – even if small – remains. cracks. The heat-affected layer generated through scrub- TYPES OF HEAT TREATMENT HEAT OF TYPES

25 ELECTRICAL DISCHARGE MACHINING Electrical discharge machining is a further technique with extraordinary results if tools with geometrically difficult Tensile stresses forms are cut out of plates. In this process the affection of Compression stresses the tool surface is small and thus less problematic. Never- Stress field after Possible crack formation theless, the unrestricted application of electrical discharge hardening (schematic) through stresses released machining is not free of risks. Larger tools always contain more or less high inherent stresses after heat treatment. During eroding this stress condition is seriously disrupt- ed; the stresses released by this can cause cracks. This is

shown schematically in Figure 25 (upper half). This risk can Stress field reduced Lower-risk finishing through stress relief cuts of the cut be considerably reduced if before heat treatment stress Figure 25 removal cuts are carried out, which are aligned to the con- Risk of cracks during electrical discharge machining of large tools tours of the tool (Figure 25 lower half). This measure leads to a considerably reduced stress field after heat treatment; the danger of crack formation during eroding is reduced.

INFLUENCE OF THE MOST IMPORTANT ALLOY ELEMENTS ON THE PROPERTIES OF TOOL STEELS (SCHEMATIC) Indirect influences, i.e. the affection of toughness through hardenability, are not considered in this scheme which merely carries orienting character.

STRENGTH HIGH-TEMPERATURE HIGH-TEMPERATURE

TEMPERING TEMPERING RESISTANCE

TOUGHNESS

THROUGH- HARDENABILITY

WEAR RESISTANCE WEAR

QUENCHING HARDNESS AFTER AFTER HARDNESS solved solved solved solved solved as alloy carbide as alloy carbide as alloy carbide as alloy carbide as alloy unsolved (carbide) unsolved

ALLOYING ALLOYING ELEMENTS Cr Si Mn Ni Mo W V Negative negative Slightly positive Slightly positive Moderately Positive positive Strongly C influence No

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Telephone + 49 (0) 6898 10 4310 Saarschmiede GmbH Freiformschmiede is a company of the Saarstahl Group.

All figures refer to standardized test samples and are given to the best of our knowledge but without further commitment. It is Saarschmiede’s belief that information set forth in this brochure is accurate. Saarschmiede makes no warranty, expressed or implied, with respect thereto and disclaims any liability from reliance thereon. All data are subject to change without prior notice.