Investigation of surface hardening effect on the local mechanical properties of the cutting tools working edge
In this work the mechanical properties of the punching die cutting edge were investigated by instrumental indentation. The near-surface layer of punching dies is modified in the process of mechanical action with a milling tool and local heating by a laser. Profiles of the hardness on depth have been constructed using the method of automated mapping. It was found that the processing of the tip by local laser heating leads to an increase in the hardness of the alloy by a factor of 1.5 in the region of about 40 μm.
Local hardening of critical areas of the surface that are exposed to increased mechanical stress is one of the most important stages in production of tools, units and parts of machines and mechanisms.
Basically, surface hardening methods can be divided into two main groups:
hardening of a product without changing the chemical composition of the surface but with changing its structure. Hardening is achieved by surface hardening, surface plastic deformation and other methods;
hardening of a product by changing the chemical composition of the surface layer and its structure. Hardening is carried out by various methods of chemical-thermal treatment and application of protective layers.
Surface deformation is one of the simplest ways to improve the surface strength characteristics. The main purpose of surface plastic deformation is to increase the fatigue strength by work hardening the surface to a depth of 100–300 μm. The types of such processing are shot blasting, roller processing, micromilling, relief knurling, etc.
The laser is actively used for local hardening of tool steel products. Laser surface treatment increases hardness, resistance to corrosion and mechanical wear . Laser hardening is a process similar to conventional through hardening,but it is carried out in a limited area and an order of magnitude faster. The laser heats the area to the phase transition temperature without melting the metal. Upon cooling, austenite turns into martensite, which gives an increase in hardness and the formation of residual compressive stress on the surface associated with an increase in volume [1, 11, 12].
In addition to laser heating, surface hardening is also influenced by machining during edge shaping. Deformation and heating, when interacting with milling tools, can also change the surface properties of materials [13–15].
Local hardening of a narrow cutting edge is widely used in production of punching dies of the punching machines. Punching means cutting a workpiece around the outer contour by hitting a die on a sheet material: paper, cardboard or polymer film. The cutting edge quality is one of the main parameters that determine reliability and productivity of the die cutting machine.
This paper presents the results of using automated algorithms for mapping hardness and elastic modulus to study the local mechanical properties of a surface-hardened tool using mechanical micro-milling and local laser heating.
In the course of sample preparation, hot pressing of punching dies samples was carried out on a MECAPRESS 3 machine (PRESI, France), the material of the pressing was phenolic resin (hardness 90 Shore D). Samples were filled perpendicular to the sample plane and perpendicular to the cutting edge.
The sample surface was processed on the polishing and grinding equipment manufactured by Struers (Switzerland). The roughness after polishing was controlled by three-dimensional images of the sample surface (Fig.1) obtained with a S neox optical profilometer (Sensofar, Spain). The color and brightness in the image shows the height and unevenness of the surface. Sample blockages are visible at the edge of the sample. The roughness of the surface Ra of the sample according to the optical profilometer does not exceed 1 nm.
Measurements of hardness and modulus of elasticity (Young) were carried out on a nanohardness tester "NanoScan-4D" (TISNUM, Russia) [16–20]. Measurements were carried out by the instrumental indentation method in accordance with GOST R 8.748-2011, also known as the nanoindentation method . This method is based on indenting a pyramidal diamond tip into the material while simultaneously measuring the depth and loading force. The analysis of the loading-unloading curve allows you to calculate hardness and elastic modulus of the material, as well as the coefficient of elastic recovery, the work of deformation and a number of other less significant parameters of the material.
Positioning of the test area was carried out using a video image of an optical microscope. Due to the high-precision sample positioning system, accuracy of the measurement coordinates is no more than 1 μm. In order for adjacent measurements not to influence each other, pricks should be located at a distance of more than 3 sizes of imprints from each other.
The measurement of hardness profiles was carried out using automated algorithms that allow large series of measurements to be performed without participation of the operator [22, 23].
RESULTS AND DISCUSSION
Carrying out a series of tests in automatic mode with registration of coordinates makes it possible to investigate heterogeneity of the sample properties over the surface. Such studies include mapping procedures and measuring the hardness profile. Mapping is achieved by carrying out indents with the same load on a regular grid, hardness profile and in series of tests along the selected direction at a constant step. It is possible to improve a spatial resolution when measuring the hardness profile by increasing the number of tests, for example, with the help of additional rows of indents offset relative to adjacent rows.
A series of indents along and across the cutting edge with a load of 50 mN were drawn on a sample of a punching die in an automatic mode. Fig.2 shows a photomicrograph of a series of indentations after building a hardness profile No. 1 across the projection of the punching die. Figure 3 shows the corresponding profiles of hardness and modulus of elasticity calculated from the results of a series of punctures. As can be seen from the graphs in Fig.3, there is no difference in hardness in the center and at the sample edge.
Figure 4 shows a photomicrograph of a series of imprints after building a hardness profile No. 2 along the stamp. Figure 5 shows the corresponding profiles of hardness and elastic modulus.
The 0 coordinate of the abscissa of the graphs in Fig.5 corresponds to a certain point in the sample array. Increasing the coordinate means approaching the edge of the sample, i.e. to the cutting edge of the punching die. The graph (left) shows that the value of hardness increases from 5.5 ± 0.5 GPa in the depth of the sample to 9.2 ± 0.7 GPa at the edge, in the area of 40 ± 5 μm. The increase in hardness is also clearly visible in the size of the imprints in the photograph (Fig.4): the size of the imprints decreases with a constant loading force. At the same time, the observed decrease in the value of the elastic modulus by 10% is insignificant and may be associated not so much with a change in the properties of the material as with the local inclination of the surface in the region of the sample edge. It is known that such a slope always occurs at the edges during mechanical polishing and sample preparation, and calculation of the elastic modulus by the instrumental indentation method is extremely sensitive to the slope of the loading curve, which, in turn, depends on the angle between the axis of the indenter and the plane of the sample.
In this work, in the automatic indentation mode, a series of measurements of hardness and elastic modulus was carried out in order to construct a profile of the distribution of mechanical properties near the cutting edges of cutting dies. The investigated edges were subjected to surface hardening by mechanical micro-milling (lateral side of the stamp) and by local laser heating (cutting edge).
It is shown that automated mapping of the marginal area makes it possible to identify local changes in the mechanical properties of the near-surface layer (hardness and elastic modulus) with a spatial resolution of about 3–5 μm). Spatial resolution depends on the selected indentation load and, accordingly, the distance between adjacent measurements, taking into account the criterion at which adjacent measurements do not influence each other. It is shown that it is possible to increase the spatial resolution by measuring several series of punctures, displaced relative to the neighboring ones by a distance less than the distance between the punctures.
The obtained dependences of hardness on the distance to the cutting edge make it possible to determine the depth of the near-surface layer which properties were modified as a result of processing. In particular, this work shows that micromilling of the die walls with a cutter does not lead to its hardening, while local laser heating significantly increases the edge hardness compared to hardness in the bulk of the material (from 5.5 ± 0.5 GPa to 9 , 2 ± 0.7 GPa). In this case, according to the obtained hardness profiles, it can be determined that the near-surface layer thickness wherein the properties change equals 40 μm. This information can be used in the development of technological processes for production of tools as a feedback for selection of the operating modes of laser installations for local heat treatment. ■
The work was carried out within the framework of the state assignment of the Federal State Budgetary Scientific Institution TISNCM for 2020.