High speed impact cutting (HSIC) of advanced high strength steel 42SiCr under exploitation of adiabatic shear bands

Shear Cutting of Advanced High-Strength Steels poses technological challenges due to the substantial mechanical loads imposed on cutting tools, leading to elevated wear rates. A strategy for cutting high-strength materials involves the utilization of high-speed impact cutting (HSIC), wherein component separation occurs along a locally adiabatically heated shear band, resulting in reduced cutting forces. The steel alloy 42SiCr undergoes heat treatments involving Quenching+Tempering (Q+T) as well as Quenching+Partitioning (Q+P) for two sheet thicknesses. This results in the formation of martensitic microstructures with varying retained austenite content, as determined through X-ray Diffraction (XRD). Subsequently, the heat-treated steel samples are subjected to tensile testing for mechanical property evaluation, revealing ultimate tensile strengths exceeding 1500 MPa and fracture elongations ranging from 2 % to 12 %. Following this, the material is subjected to HSIC using the AdiaPress Adia 7 machine, employing predefined cutting energies. It is observed that both Q+T and Q+P-treated materials can be successfully cut using HSIC, although distinct cutting edge morphologies are evident. Optical examinations of the cut edges, conducted through top-view and cross-sectional analysis using Scanning Electron Microscopy and 3D laser scanning microscopy, confirm the presence of adiabatic shear bands and discrete zones.


Introduction and state of the art
Quenching and partitioning (Q+P) steels have been widely investigated in terms of mechanical properties, fabrication routes and processing, since Speer et al. [1] first reported the partitioning of carbon from supersaturated quenched martensite into retained austenite (RA) in 2003.As a result of their high strength and ductility, Q+P steels represent the third generation of advanced high strength steels (AHSS) for the automotive industry [2,3].The reason for their outstanding properties is the initial thermal treatment of the steel, resulting in a fully martensitic structure with increased RA content [4][5][6][7].Q+P steels gain their high strength properties from the martensitic content and the enhanced ductility by RA of up to 20 % of phase content.Plastic deformation can be absorbed by the RA which transforms simultaneously to martensite and strengthens the material.The mechanical processing of Q+P steels is strongly limited by its high strength.When considering shear cutting, the tools are subjected to increased wear, by which the tool life quantity is significantly reduced.As an enhancement of the conventional shear cutting process, high-speed impact cutting (HSIC) emerged.With the help of a defined energy input which accelerates the punch up to 10 m/s, the blanking material is deformed very fast along the cutting line which heats the material up by several hundred Kelvin within 100 µs [8].Adiabatic heating of the shear zone occurs in a range of less than 100 µm [9][10][11][12].About 95 % of the cutting energy is converted into heat [13] which softens the material and enables the cutting process.Winter et.al. [9] investigated the temperature rise during adiabatic cutting of a press hardening steel based on a thermomechanical model approach.Yang's studies in orthogonal cutting [14] show enhanced shear band formation when low thermal conductivity, low heat capacity, low strain rate sensitivity, and high yield stress occur in the material.Adiabatic shear bands begin when a critical, material-specific deformation rate is reached and sufficient energy is available for plastic dissipation [15].In order to activate the adiabatic shear bands for the cutting process the punch has to penetrate the component less than 10 % of the blank thickness.As a result of the low penetration of the punch, spring back aspects are negligible in comparison to cutting with lower punch velocities [13].The HSIC process thus results in improved cut surfaces, nearly free of burrs, high contouring accuracy and the reduction of shear cutting forces [16,17].

Experimental procedure 2.1. Chemical composition
The HSIC tests were carried out on the experimental cast of 42SiCr steel, that was investigated for the sheet thickness of 1.8 mm and 3.6 mm after rolling.The chemical composition of the material was analyzed by OES Bruker Tasman Q4 (see Table 1).

Initial heat treatment
The base material was exposed to three different variants of heat treatment in order to set various mechanical properties (Figure 1).The thermal treatment is based on preliminary investigations and literature values on Q+P [4][5][6][7].After the austenitization subsequent quenching with > 30 K/s is performed in a heated salt bath.The temperature of the salt bath is set on a temperature level between the martensite start temperature (M s ) and martensite finish temperature (M f ) of the material.The partitioning process takes place in the heated salt bath, at which the dissolved carbon atoms diffuse out of the supersaturated martensite into the intermediate space between the martensite needles and stabilize RA.According to the literature [4][5][6][7], the diffusion-controlled process takes between 15 and 20 min.The quenched and tempered (Q+T) material state is quenched to temperatures below M f and subsequently tempered, as in conventional tempering and annealing processes.The routes Q+P-1 and Q+P-2 are quenched and partitioned with divergent temperatures and intervals (Table 2).

Tensile testing of heat treated base material
In order to evaluate the mechanical properties of the base material after heat treatment, all tests were performed by the universal testing machine H&P Inspekt 150 according to DIN EN ISO 6892-1.The tensile tests were carried out for all heat treated conditions and both sheet thicknesses at room temperature and a testing velocity of 0.5 mm/s.The initial gauge length was L 0 = 40 mm.

XRD analysis of heat treated base material
After the as-received condition had been heat treated, a phase analysis was performed by means of X-ray diffraction to identify the RA content.The samples were measured on the crosssectional area.Quantitative analyses were performed according to Rietveld using the Siemens D5000 X-ray diffractometer.

High speed impact cutting experiments
The HSIC process was carried out on an AdiaPress Adia 7 machine, which is driven by a hydraulic cylinder and can provide cutting energies stepwise between 1.7 and 7 kJ.A tool for shearing with a circular cutting punch was used.The diameter of the active tool parts were 30.0 mm for the punch and 30.2 mm for the die.This resulted in a cutting clearance related to the sheet thickness of 5.6 % for a base material of 1.8 mm and 2.8 % for a base material of 3.6 mm sheet thickness.Prior publications indicates a suitable clearance for HSIC between 2 to 4 % of the sheet thickness [8].This clearance is significantly higher compared to fine blanking (∼ 0.5 %), but lower compared to conventional blanking (5 -15 %) [18].For the sheet thickness of 1.8 mm, cutting energies of 1.7, 2.5 and 3.5 kJ were performed.The test of 42SiCr steel with the sheet thickness of 3.6 mm was carried out with cutting energies of 2.5 to 4.5 kJ, whereby the impact energy was increased by 1 kJ stepwise.In each possible parameter combination, a minimum of 3 experiments were conducted.

Optical evaluation of cut material
After the HSIC tests, the quality of the cut surfaces at the cut out part and at the sheet metal strip was investigated, using optical microscope OLYMPUS GX51, SEM LEO 1455VP and ZEISS NEON 40EsB as well as 3D laser scanning microscope Keyence VK-9700.The cut surfaces were analyzed in front view and in cross-section (Figure 2).For the cross-sectional analysis, the material was polished and etched with picric acid for 50 seconds.

Mechanical properties of heat treated basic material
With the help of tensile tests of heat-treated sheet materials of 1.8 mm thickness, the highest tensile strength (R m = 2040 MPa) and the smallest ultimate strain (A 40mm = 7.5%) were measured with Q+T treated specimens (Figure 3 left).Compared to Q+T, the Q+P treatment slightly reduces the tensile strength but increases the ultimate strain almost by the factor of 2. Thus, the maximum measured tensile strength and ultimate strain for Q+P-1 is R m = 1720 MPa and A 40mm = 13.5 % and for Q+P-2 R m = 1950 MPa and A 40mm = 10.5 %.Compared to sheets of 3.6 mm thickness (Figure 3 right), 1.8 mm sheets tend to have an increased yield stress by up to 200 MPa but an about ∆A 40mm = 2 % reduced ultimate strain.For Q+P-2, the treatment of 3.6 mm thick material provides a maximum tensile strength of R m = 1680 MPa and an ultimate strain of A 40mm = 12.5 %.Q+P-1 exhibits a tensile strength and an ultimate strain of R m = 1560 MPa, respectively A 40mm = 15 %.For all three individual heat treatments, there is a deviation in the stress-strain curve during consecutive tensile tests, despite an identical heat treatment.This can be attributed to the use of the experimental cast 42SiCr and the discontinuous aspect of the heat treatment.Additionally, it is possible that the mechanical properties are highly sensitive to the heat treatment and RA content.

Phase composition and microstructure after Q+P and Q+T treatment
The phase analysis of the material provides information on the RA content.Measurements were performed on surfaces that were cut by saws or water-jet, ground and polished.It can be assumed that the RA on the surface turns into martensite due to the mechanical processing.
As given in Figure 4, there is a significant difference between Q+T and between Q+P treated samples.For example, the proportion of RA is less than 2 % for 1.8 mm thick sheet metal after Q+T treatment and less than 3 % for 3.6 mm thick material.In contrast, the fraction of RA of Q+P-1 heat-treated sample is much more pronounced.A content of 9 % can be determined for 1.8 mm thick material and 9.5 % for 3.6 mm thick material.The highest RA content was found in Q+P-2 treated samples ranging from 9.5 % for 1.8 mm thick material to 12.2 % for 3.6 mm thick material.So, the RA content increase for 1.8 mm material is between ∆RA = 0.5 -2.7 % lower than for 3.6 mm thick material.The influence of the preparation is also verifiable, since all sawn samples have lower RA content than samples that were ground and polished after water-jet cutting.As illustrated in Table 3, after the respective heat treatments the specimen reveal different martensitic microstructures in the inner material.The images were taken by optical microscope.After a Q+P-1 treatment of the 1.8 mm thick material another phase is evident in the form of light grains that do not react with picric acid.According to the Q+P treatment and the analyzed austenite content, it is supposed to be grains of RA.

Cutting experiments with HSIC
The HSIC experiments for all 3 heat treatment conditions and both sheet thicknesses were carried out.The smallest cutting energy input was high enough to cut the samples.Q+P and Q+T-treated 1.8 mm sheets were cut with 1.7 kJ successfully just like 3.6 mm thick material at 2.5 kJ.No evident differences were found at the optical evaluation of the cut surfaces in regard to the heat treatment.

Microscopic analysis of the cut surface in cross section
The investigated cross section from the cut surface into the middle layer of the material can be divided into three distinct zones according to the distance from surface. Figure 5 illustrates the cross section of 3.6 mm Q+P-1 treated cut-out material.The undeformed martensitic base material in zone 1 does not show any obvious deformation.In contrast, severely deformed martensite needles were formed in zone 2. The curvature of the martensite needles amounts to 90 °.The average diameter of the zone is 10 to 15 µm.Bordering to this layer of strongly curved martensite needles, the highly deformed zone 3 with a thickness of 8 to 15 µm is present.It differs significantly from the underlying structure.Between zones 2 and 3, a sharp transition of the structure can be detected.The curved martensite needles from zone 2 in Figure 5 are shown in detail in Figure 6 for similar HSIC cut material.Obviously, the needles do not break due to the shear process (6 a).The deformation is endured by the martensite without obvious fracture.Zone 3-grains of similar HSIC cut material are spherical and slightly stretched in the direction of deformation (Figure 6 b), whereby a length up to about 200 nm and a height between 100 and 150 nm was determined.Thus, the area is ultrafine-grained.Delamination of zone 3 is illustrated in Figure 6 c.

Microscopic analysis of the cut surface in front view
SEM images demonstrate the characteristic formation reported previously [8].The fracture zones in the front view are exemplified in Figure 7 for a 3.6 mm sheet after Q+P-1 treatment and HSIC with 2.5 kJ.The upper edge of the cut surface shows a rollover height ranging from 80 to 150 µm, followed by the burnish.Consistent with prior HSIC research [8], the burnish height accounts for 4 % of the sheet thickness.An adjacent dimple fracture area, 50 to 150 µm thick, is visible in the cutting direction (Figure 7).Below the dimple fracture zone, a broad smooth area ascends towards the opposite sheet surface, accumulating in a plate-like manner up to the tear-off at the burr.The cut surface morphology is similar for the 1.8 mm sheet thickness, with a less pronounced extent of the plate-like structure and a smaller increase of the even surface towards the opposite edge.The rollover height is lower in comparison to 3.6 mm sheet thickness, with the burnish cut accounting for 5 % of the sheet thickness.The dimple fracture below the burnish is smaller than in the 3.6 mm thick sample.Once again, a region of reduced roughness is evident below the burnish with a subsequent dimple fracture.

Topographic measurement of the cut surface
For both Q+T and Q+P treatments, no significant differences in the cut surface morphology are observed for their respective sheet thicknesses.To assess the shape of the cut surface of cut-out parts, topographical measurements of the 3.6 mm thick sample after Q+P-1 treatment and HSIC with 2.5 kJ were performed using a Keyence laser microscope, displaying morphology similar to optical microscopy and SEM.The height profile, enlarged by 200 %, highlights key areas.In Figure 8, the burnish cut portion forms a flat surface on the left.The subsequent dimple fracture is followed by a rapid fall-off of the cut surface, with the adjacent area exhibiting an increased profile.The strong plate-like elevation, around 25 µm relative to the surrounding material, is visible in the cross-sectional view in Figure 8.In comparison to the 3.6 mm thick material, the 1.8 mm thick material exhibits a similar but less pronounced cut morphology.Underneath the burnish, the cut surface displays a less distinctive dimple fracture, followed by a flat and even area.A local elevation of approximately 10 µm forms a structure akin to the plate-like structure observed in the 3.6 mm thick material (Figure 7).This shape has been previously observed for AHSS and described as an S-shaped curve [12].

Discussion
Investigations show that 42SiCr sheet metal achieves desired properties via applied heat treatments.Interrupting the cooling process and partitioning between M s and M f leads to higher ductility compared to quenching below M f with subsequent tempering.Q+T-treated material exhibits higher tensile strength than Q+P-treated 42SiCr.Despite different properties observed through testing and energy input in HSIC, the cut surface morphology remains similar.Significant differences arise when comparing sheet thicknesses, possibly due to the different sheet thickness related cutting clearance.Metallographic analysis of cut surfaces reveals a threezone structure, including undeformed martensite (zone 1), strongly curved martensite in shear direction (zone 2), and ultrafine-grained equiaxial microstructure towards the cut surface (zone 3).Zone 3 likely contains adiabatic shear bands for deformation during the shear process.
Examining the front view, a multi-level structure is evident, with a small proportion of rollover and burnish heights.A dimple fracture surface adjacent to the burnish suggests ductile fracture behavior.Below it, the cut surface deepens towards the base material and increases towards the burr on the opposite sheet surface.A zone of reduced roughness followed by plate-like structures aligns with the adiabatic shear bands in the cross section.SEM images confirm separation through the shear band and at the boundary between highly curved martensite (zone 2) and the adjacent shear band (zone 3).See Figure 9 for a schematic representation of the cut surface morphology based on metallographic analysis.

Conclusion
From the investigations it could be determined that the performed heat treatments lead to Q+P properties.The examined steel can be cut by HSIC with low energy.The examination of the surface showed a multi-zone structure in front view and in cross-section.The cross-section of the cut surface offers a 3-zone structure consisting of a zone of undeformed martensite, a second zone of strongly deformed martensite needles and a third zone of ultrafine equiaxial microstructure.The fine-grained structure is only visible as a narrow band in certain areas of the surface and is defined as adiabatic shear band.In front view of the cut surface, the shear bands are plate-like structures in the direction of the burr.Topography measurements support this statement, as plate-like structures of up to 25 µm stand out from the surface.From this a basic cut surface morphology model for adiabatic cutting can be developed.There is a need for further investigations on how the adiabatic cutting process will be influenced by increasing RA contents and what effects this will have on the microstructure at the cut surface.Furthermore, it is necessary to demonstrate how a cut surface would appear in conventional shear cutting under the same material and die clearance conditions.Wear studies on cutting tools during HSIC and conventional cutting of Q+P steel are crucial to quantify the advantages of HSIC.

Figure 1 :
Figure 1: Schematic route of heat treatment sheet metal strip cut out part area of front view measurement area of front view measurement location and direction of cross-sectional investigation location and direction of cross-sectional investigation

Figure 2 :
Figure 2: Cross-sectional view on cut sheet metal strip and cut out part

Figure 3 :
Figure 3: Stress-strain curve after Q+T and Q+P for sheets with 1.8 mm (left) and 3.6 mm (right) thickness

Figure 5 :
Figure 5: Cross section of Q+P-1 cut-out part with 3.6 mm sheet thickness after HSIC with 2.7 kJ, SEM 1000 x

Figure 7 :
Figure 7: Front view on HSIC cut surface 3.6 mm thickness after Q+P-1 and 2.5 kJ cutting energy SEM 20 x (left) and detailed view 140 x (right)

Figure 9 :
Figure 9: Schematic sketch of cut surface morphology and microstructure after HSIC

Table 2 :
Heat treatment routes for sheet thickness 1.8 mm and 3.6 mm