Hot Working Tool Steel with Bainitic Microstructure—The Mechanical Properties at Elevated Temperature

Usually, tool steels are used in quenched and tempered conditions. Due to the phase transition from austenite to martensite and the volume change during the transformation, controlling distortion might be challenging. A newly developed steel focusing on a bainitic structure, even for bigger dimensions, shows a lower hardness at ambient temperature than conventional hot working tool steels but with a lower temperature dependency. Therefore, at service temperature, the mechanical properties are comparable to established grades. Heat treatment is simplified by a simple austenitization and cooling process without special requirements on the cooling rate. Due to a generally lower cooling rate and lower hardness compared to a quench and tempering process, the risk of cracking is reduced while machinability is improved. With the combination of good weldability and rather low hardness after rapid cooling, this grade can also be processed in additive manufacturing and is well suited for a hybrid process of conventional and additive manufacturing.


Introduction
The production and use of tools for forging is a multi-objective optimizing problem with usually multiple involved processes, starting from steel making with intermediate heat treating and machining steps up to final thermochemical surface treatment. [1,2] It has to be stated that performance improvement of new steel development not always entered the market due to missing or too small economic effects, e.g., small production lot sizes in forging.
To overcome this kind of over-engineering, a project aiming at a short process chain with a limited amount of alloying was started. The aim of the project was to optimize the economics of the complete process by reducing alloying cost, shortening the process chain from steelmaking to tool production, but not aiming for infinite tool life.
Actually, most of the release tests like hardness and impact testing are done at ambient temperature, whereas the inservice temperature is much higher. Not focusing on a high hardness and low machinability at ambient temperature, but more on the properties at higher temperatures leading to a concept based rather on a bainitic microstructure than on a martensitic one.

Alloying Concept
Depending on the chemical composition and cooling rate, bainite is a structure very similar to a martensitic structure after quenching and tempering with similar mechanical properties. A homogeneous structure is beneficial for fatigue because of the non-localized plastic micro deformation. In contrast, a mixed structure with soft ferrite and perlite is much easier for the machine. To optimize the economics of tool steels, a steel with bainitic structure and mechanical properties in the range of quenched and tempered steel could be a candidate as shown in Figure 1.
The avoidance of a quenching and tempering process step leads to a bainitic microstructure. To achieve this structure there exist a few rules of thumb: [2][3][4][5][6] 1) The lower the carbon content, the more ductile the material but also the hardness decreases. Bainitic steels usually show a carbon content between 0.05% and 0.35%. 2) Silicon decreases the tendency of carbide precipitation and is promoting the formation of soft ferrite, increases strength by solid solution hardening but decreases toughness. In this development process it was limited to a maximum of 1.3% 3) A concentration of Manganese between 1.0% and 2.5% has a small effect on the transformation behavior and solid solution hardening. A beneficial effect of up to 3% Manganese on tool wear is reported by Pfahl. [7] 4) Chromium with an amount below 2.5% improves through hardenability and forms carbides. 5) Molybdenum reduces strongly the formation of ferrite and perlite. The formation of carbides is slow compared to chromium. This element is regarded as the key element for achieving a bainitic microstructure even for bigger dimensions which are typical for tool steel applications. 6) Boron suppresses the austenite to ferrite or perlite transformation if in a solid solution. 7) Other elements like V and Nb may be added mainly for suppressing extensive grain coarsening and also suppress the formation of ferrite and perlite. [8][9][10] DOI: 10.1002/srin.202200329 Usually, tool steels are used in quenched and tempered conditions. Due to the phase transition from austenite to martensite and the volume change during the transformation, controlling distortion might be challenging. A newly developed steel focusing on a bainitic structure, even for bigger dimensions, shows a lower hardness at ambient temperature than conventional hot working tool steels but with a lower temperature dependency. Therefore, at service temperature, the mechanical properties are comparable to established grades. Heat treatment is simplified by a simple austenitization and cooling process without special requirements on the cooling rate. Due to a generally lower cooling rate and lower hardness compared to a quench and tempering process, the risk of cracking is reduced while machinability is improved. With the combination of good weldability and rather low hardness after rapid cooling, this grade can also be processed in additive manufacturing and is well suited for a hybrid process of conventional and additive manufacturing.

Transformation Properties
A typical analysis of grade Thermodur 2322 is given in Table 1 and the corresponding continuous cooling transformation diagram is shown in Figure 2. The samples are taken from a regular production and within the range of the internal specification of the electric arc furnace (EAF) steel plant. The other elements in Table 1 are unavoidable residual elements in a standard EAF process. The temperatures for martensite start M s and finish M f are estimated.
The continous cooling transformation (CCT)-diagram of grade 1.2322 shows a wide range of cooling rates with a nearly complete bainitic microstructure without the formation of ferrite or perlite.
Usually, the maximum size to achieve a completely transformed bainitic microstructure is limited to values below 100 mm. For bigger sizes, a mixture of martensite in the subsurface region and a certain amount of ferrite in the center are expected. For evaluation of industrial heat treatment, a block with the dimensions 1010 mm Â 250 mm Â 6000 mm was air cooled after forging, cut into two pieces, and subsequently the hardness was measured. For the dimension of 250 mm, which is the most important one for controlling the cooling rate, a small decrease of hardness in the center of the material has to be stated as shown in Figure 3.
The structure just below the surface and mid-radius was pure bainite whereas in the center of the block an amount of less than 1% ferrite beside bainite was certificated.

Mechanical Properties
For controlling the heat treatment of tool steels, testing of material properties is focused on hardness measurements at ambient temperature. For the release test of the material, Charpy impact testing is also common practice. These tests are aiming on good in-service properties like: 1) Avoidance of cracks during heat treatment (QT); 2) Wear resistance; 3) Mechanical fatigue at elevated temperatures; 4) Thermal shock resistance.
In this work, we are focusing on the mechanical properties at elevated temperatures.

Tensile Strength at Elevated Temperatures
For the evaluation of the mechanical properties at higher temperatures, tensile strength and impact toughness are tested. Figure 4 shows the tensile strength according to ISO 6892-2 of grade 1.2322 compared to grade 1.2714. The grade 1.2714 is often used for hot working applications due to its low costs. With increasing temperature, the higher hardness of 1.2714 is overcompensated by the higher temperature stability of grade 1.2322.
Usually [11] steel grades with a very low carbon content are not regarded as suitable for nitriding because of crack formation due to a high hardness gradient from the surface layer to the bulk material. Comparing the strength and especially the yield strength, 1.2322 should be comparable to 1.2714 with a nitrided surface.
Comparing the tensile properties of 1.2322 referencing higher alloyed 1.2343 we find the same tendency as shown for 1.2714. The higher the temperature, the less the difference between the two grades as shown in Figure 5. For the yield strength, the crossover takes place between 550 and 600°C. Not surprisingly, the 1.2343 performs better than the 1.2322 but at higher costs, higher sensitivity for cracks, and lower machinability in a non-soft-annealed condition.

Toughness Properties al Elevated Temperatures
The toughness of the specimen is tested as a standard Charpy-V impact test according to ISO 148 and impact bending SEP 1314 between 400 and 600°C representing the in-use condition. The difference between the two types of specimen besides a small difference in the specimen dimensions is the existence of a notch in the Charpy-V specimen and the absence of a notch in the impact-bending one. Even at the same level of yield strength, 1.2322 performs better than 1.2714, as shown in Figure 6. This is indicating a higher resistance of the bainitic grade against crack formation than for the quenched and tempered grade.

Heat Checking Resistance
The resistance against thermal shock (heat checking) was tested with induction heating and fast cooling as described elsewhere. [12] The temperature profile used in this work is similar to the test applied by Norwood et al. [13] for die casting. The evaluation is done by counting the cracks, measuring the crack length, and calculating the total length of the cracks after a given   number of thermal cycles. As a reference different steel grades are tested before being used. The results are summarized in Figure 7.
The lower strength and the high impact toughness lead to an outstanding performance in this heat-checking test. Ebner et al. [14] stated a detrimental effect of bainite on the heat-checking sensitivity of a quenched and tempered hot-work tool steel with a mixed microstructure of martensite and bainite. With a homogeneous microstructure of bainite, the negative effect of bainte on heat-checking vanishes as shown in this work.

Additive Manufacturing
Additive manufacturing isn't actually a process for the mass production of components. The production of tools is usually in small lot sizes down to lot size one. The drawbacks of additive manufacturing are the limitation to small dimensions, the low number of available "easy to use" steel grades, and the long process time. Sometimes a preheated powder bed is necessary [15,16] but the maximum temperature is often limited to 200°C. Taking the material properties described before into account, the manufacturing of steel powder and processing of the powder made of 1.2322 was tested in a publicly sponsored project called AddSteel (EFRE-0 801 141). [17] The process of additive manufacturing is a welding process layer by layer with a resulting rapid heat up by a laser up to melting temperature and fast cooling due to the small mass of the welded spot compared to the rest of the components already built. [18] This results in the case of austenite to martensite and vice versa transformation, [19] which leads to mechanical stress between cold and hot layers as visualized in Figure 8.
More critical than the stress due to the transition from austenite to martensite for crack formation seems to be the stress between the hot and cold layer by rapid heating because of the high hardness of both layers.
Within the AddSteel project, it was decided to produce a demonstrator as fast as possible. The final result was the production of a hydraulic component as shown in Figure 9.
Usually, building rates above 15 cm 3 h À1 are difficult to achieve. It has to be mentioned that due to the high energy input and the extensive formation of fume extra countermeasures have to be taken. The mechanical properties of additive-manufactured     samples are similar to conventionally produced ones, as shown in Figure 10.

Conclusion
Changing the focus from hardness at ambient temperature to that at usual in-service conditions a hot work tool steel with a bainitic microstructure was developed. Yield strength at elevated temperature, toughness at elevated temperature, and the heat checking resistance are promising. The process chain can be shortened due to the low hardness at ambient temperature, which results in a better machining capability than actual standard grades.
The capability of additive manufacturing without preheating makes the production of bigger parts by this process possible. Even a hybrid component, partly produced by machining and adding functional parts by additive manufacturing is rather simple without additional heat treatment.
The low carbon content of this grade leads to an expectation of a lower wear resistance than other grades [20] like 1.2714 or 1.2343/1.2344. As reported by Doege et al. [21,22] a higher temper resistance improves tool wear. The low carbon content doesn't seem to be an obstacle for nitriding to improve wear resistance comparing the mechanical properties of 1.2714 and 1.2322. Tarkany [11] shows the combination of strength, toughness, and wear resistance depends strongly on the application. Therefore, grade 1.2322 may be a further useful solution for hot forging tools. The wear resistance needs, however, to be investigated to verify the above-mentioned anticipated results.