The variation of the hardness of high-strength steels depending on the microstructures specific of tempering

The properties of steel depend on its chemical composition and structure. The modification of the structure in order to change the properties in a certain way is not only the main purpose of heat treatment, but also the only purpose, because the heat treatment regime influences the variation of properties through the variation of the steel structure. It is only by correlating the variation in structure with hardness that we can answer how heat treatment influences hardness, which also depends on the stress state. The tempering results are mainly influenced by the heating temperature, holding time and less sensitively by the tempering cooling rate (air, oil) as shown in the research carried out and presented in this paper. Tempering reduces hardness, internal stresses, the amount of residual austenite and increases elongation, necking and resilience at the expense of strength. The characteristics of tempering structures are clearly superior to equilibrium (annealing) structures in that the degree of dispersion of the structure is greater and the shape of the carbides and constituents is fine globular.


Introduction
Today, worldwide, research is being carried out to develop new steels with the highest mechanical, physical and chemical characteristics.
Areas such as space, sea and ocean research and other common areas require the use of steels superior to those generally used in the machine-building industry.The need for these steels with superior strength characteristics results from the harsh operating conditions, the high durability of the various assemblies and sub-assemblies made from these steels, and the possibility of reducing their weight, which is of great importance, especially in the aeronautical industry [1], [2], [3].
In this field, light alloys are often outdated due to the increasing need for high heat resistance, thus explaining the interest in high and very high strength steels [4].
The studies which have been carried out in the field of the development of the steels necessary for the execution of massive parts in aircraft construction concern, in particular, the realisation of parts whose fatigue strength is as high as possible, since, in most cases, it is almost impossible to determine the conditions where the principal stresses are perpendicular to the direction of orientation of the grains [1].It follows, therefore, that the toughness and hardness of the steel must be important.Thus, high and very high strength steels have been developed, combined with appropriate toughness, structural steels capable of achieving strengths greater than 140 daN/mm 2 [5].
Today, landing gears are made of structural steels with strengths from 190 daN/mm 2 to 207 daN/mm 2 , and designers are demanding even higher strength steels, around 340 daN/mm 2 , so that the need to substantially reduce aircraft weight is made possible by using such steels [4].
Several methods are used to achieve this goal, but in this paper we have chosen to make attempts to improve the properties of classical alloys.

Characterisation of high strength steel produced in Romania
In our country, the most important brand of steel intended for the construction of landing gear is 15VMoCr14X, which belongs to the Cr-Mo-V class of steels, intended for the manufacture of parts with high strength requirements, working in temperature conditions from -75 o C to + 500 o C [5].
The chemical composition of 15VMoCr14X steel is shown in Table 1 [6].The delivery conditions and consequently the characteristics that the company producing the steel must guarantee are shown in Table 2 [6].From the analysis of the delivery states and the conditions that high and very high strength steels intended for the aeronautical industry must ensure, the fatigue strength and toughness, both at normal and low temperatures, stand out in particular.These mechanical characteristics can be significantly improved by applying heat treatments according to the TTT diagrams of the steels concerned, resulting in different grain structures and sizes depending on the heating temperature, heating time and cooling rate [5], [6].
Thus, the properties of the steel depend on its chemical composition and structure.The modification of the structure is not only the main purpose of heat treatment, but also the only purpose, because the heat treatment regime influences the variation of properties through the variation of the steel structure.
Only by correlating the variation of the structure with the resilience, it is possible to answer how the heat treatment influences the hardness, which also depends on the stress state [7], [8].

Determination of hardness and resilience of steel at normal, high and low temperatures
The numerical values of the mechanical characteristics of a steel are determined by a series of tests which are differentiated by the type of macroscopic deformation generated (tensile, compression, torsion, bending, hardness, etc.), the mode of application of the load (static, dynamic, variable in magnitude and direction), the test temperature, etc.The most common tests are static tensile tests, dynamic impact bending tests and static hardness tests.The experimentally determined mechanical characteristics provide information on the conditions under which the tested steel can work in service and its maximum stress limits [9][10][11].
The mechanical tests are also used to control the quality of the products produced and the transformations which have occurred in the steel during the processing to which it has been subjected.For the hardness and resilience tests, samples were taken from 80 mm rolled billets.These samples were forged on a 250 kg hammer at the following dimensions: 30 x 250 mm; 25 x 250 mm; 18 x 250 mm.The bars were then subjected to the heat treatments indicated in Table 6.The hardness of the structure obtained after hardening is determined, on the one hand, by the proportion of martensite in the structure and, on the other hand, by the hardness of the martensite.In turn, the hardness of the martensite depends almost exclusively on its carbon content (Figure 1) [5].Alloying elements increase the hardness of martensite only slightly (by 1 to 2 HRC units for the same carbon content).
Table 7. Comparative hardness values for semimartensitic structures As an example, Table 7 compares the hardness values for semi-martensitic structures (structures with 50% martensite + 50% non-martensitic constituents) of alloyed and non-alloyed steels with the same carbon content.
By statistical processing of the experimental results it was possible to determine the following formula for calculating the hardness of martensite as a function of the chemical composition of the steel (assuming that both carbon and all alloying elements dissolve completely in austenite on heating, so that its composition is identical to that of the steel) [5]: HV100M = 127 + 449C + 27Si + 11Mn + 8Ni + 16Cr + 21lg vcooling (1) where: -C, Si, Mn, Ni, Cr are the concentrations of the respective elements in weight percent; -vcooling is the cooling rate in o C/h.
In the annealing heat treatment, the samples were cooled with the furnace to provide the forged bars with a low and uniform cooling rate in order to homogenize the equilibrium-like structure.
The quenching heat treatment was carried out in a salt bath (BaCl2) plant in order to ensure the most accurate and uniform austenitization temperature on the bar section.The cooling medium used was TT 50 oil.
An electric furnace with heating in all directions (hearth, vault, side walls) was used for tempering, and cooling was done in air and oil to determine the influence of the tempering cooling medium as well as different tempering temperatures in the range 550 -700 o C on the level of mechanical characteristics.
To achieve the purpose of quenching, i.e. the hardening of steels, ideally a fully martensitic structure should be obtained in hypoeutectoid steels.On high-speed cooling austenite will transform, without diffusion, into martensite.As the end temperature of martensitic transformation drops below 0°C at carbon contents higher than 0.6%), special measures (cold hardening) must be taken to avoid the presence of a certain amount of residual austenite in the structure [8], [12], [13].
After the heat treatments, test specimens were taken from the bars in question for the KCU 300/5, KCU 300/3 resilience and hardness tests at normal temperature, high temperature, low temperature.

Figure 2. Hardness variation with tempering temperature (room temperature tests)
To determine the influence of tempering temperature and cooling medium on tempering, hardness tests were carried out near the fracture zone of the KCU 300/5 shock test specimens at room temperature and hardness variation plots were plotted as a function of tempering temperature with the values obtained (Figure 2).Since 15VMoCr14X steel is used to manufacture components for the landing gear of aircraft, which are subjected to high and low temperatures during operation, they must maintain their permissible properties in the range -75 o C ÷ +500 o C.
For this purpose, high temperature resilience and hardness tests have been carried out.From the samples tested at high temperatures, broken pieces were taken and their hardnesses were measured as a function of the tempering temperatures and the tempering cooling environment, as well as the test temperatures.With the hardness values thus determined the graphical representation was made, shown in Figure 3  Samples were taken from the broken specimens at low temperatures, from the breakage area, and prepared metallographically.These samples were etched with 2% NITAL and examined under a metallographic microscope at 500x magnification.
According to the microstructure readings, the structure is martensitic in most cases, associated with sorbite and troostite.
In Figure 6.a and b, for the temperature return to 625 o C (air and oil), a 75 -85% martensite structure associated with 15 -25% troostite was obtained, resulting in a score of 6 and 8, HRC 36 -39 hardness, resilience KCU 300/5 = 10.5 and 10.9 daJ/cm 2 , breaking strength Rm = 117 -123 daN/mm  In Figures 8 and 9, for the tempering temperatures 670 o C and 700 o C, there is a noticeable decrease in the tensile strength 106 daN/mm 2 and 103 daN/mm 2 respectively, values which do not correspond to those required by the AERO standards.14.c) grain size scores of 5 -6, resiliencies of 12 daJ/cm 2 , 9 daJ/cm2 and 6.8 daJ/cm 2 respectively, with hardnesses of 37 and 33 HRC were obtained.
Correlating the values of resilience and hardness with the microstructures over the whole tempering range, it results that the tempering range 625 -635 o C can be considered optimal for the final heat treatment of the laminated blanks that were the subject of this research.

Conclusions
The properties of steel depend on its chemical composition and structure.The modification of the structure, in order to change the properties in a certain sense, is not only the main purpose of heat treatment, but also the only purpose, because the heat treatment regime influences the variation of properties through the variation of the steel structure.
Only by correlating the variation in structure with resilience is it possible to answer how the heat treatment influences hardness, which also depends on the stress state.
The tempering results are mainly influenced by the heating temperature and less sensitively by the tempering cooling rate (air, oil) as shown in the research.
Tempering reduces hardness, internal stresses, residual austenite and increases elongation, necking and resilience at the expense of strength.
The characteristics of tempering structures are clearly superior to equilibrium (annealing) structures in that the degree of dispersion of the structure is greater and the shape of the carbides and constituents is fine globular.

3 .
.a and 3.b.a) air cooling; b) oil cooling Figure Hardness variation with tempering temperature (high-temperature tests)Alloy steels for machine construction, at low temperatures, begin to change from ductile to brittle as the temperature drops.Therefore, for the low-temperature resilience test, specimens returned to 625 o C and cooled in oil were chosen, and specimens broken at -50 o C were subjected to hardness testing.With the measured values for both broken specimens (left -right), a graphical representation of the hardness variation as a function of test temperature, tempering temperature and cooling medium was obtained (Figure4).

Table 1 .
The

Table 2 .
Steel delivery conditions and guaranteed characteristics

Table 4 .
Mechanical properties at high temperatures, as a percentage of the values from Table2