The investigation of the influence of thermomechanical treatment of the material of rotary cutter bit toolholders on its hardness

The causes of failure of the tangential rotary cutter bits of the road header during stonedrift in rocks of medium strength are analyzed in the article. It was revealed that the most typical cause of failure of cutter bits is premature wear of the toolholder (body) of the cutter bit. It is well known that the most effective way to improve the wear resistance is to increase hardness. The influence of the thermomechanical treatment of the material of the cutter bit toolholder on its hardness is studied. It was established that the thermomechanical treatment of the cutter bit toolholder material results in the increase of its hardness. It was found that the increase of material hardness is proportional to the increase of material strain intensity during thermomechanical treatment. It was concluded that the use of thermomechanical treatment can lead to the increase of both the hardness and wear resistance of the cutter bit material.


Introduction
Currently, the most advanced technique for permanent and service headways during subsurface mining is road headers, while for the mining operation -shearers that break rocks with operating devices of the cutting type.
The effectiveness of rock massif breaking and the productivity of power-loaders are defined by the engineering level of applied cutting tools. The quality of applied cutting tools affects mine costs both directly (via tool cost) and indirectly (via their replacement timing, the reduction of resource elements of transmission, actuators and other power-loader components) [1].
The exploitation of damaged cutter bits causes an increase in power and energy parameters of rock breaking. The above-mentioned reasons cause substantial costs for cutting tool replacement: depending on rock hardness and abrasiveness, they can reach up to 30% of the total cost of the tunneling work [1].
It is conventional that the runout of indexable insert holders is similar to the wearout of cutter bits having a plane face [2,3]. A finely-divided volume-compressed core built up in the contact zone of the cutting tool and a rock prevents further movement of the cutter bit ( Figure 1). Since the cutter bit does not stop moving, and the openness and cleavage of a rock massif are not sufficient for continuous cutter bit movement without crush evacuation, a part of the pressure bulb 'utpours' at a high speed along the cutter bit face towards the free face. Due to high stress of the fragmented rock in the pressure bulb, its 'outpouring' through a narrow gap is accompanied by significant friction on both the undecayed rock and the cutter bit (figure 1, c). In Figure 1, it is shown that most of the time, the hard alloy insert and the bolster of the shoulder of the rotary cutter bit toolholder interacts with the rock. Thus, rock cutting with the rotary cutter bit causes prompt appearance and destruction of the pressure bulb resulting in tool wear. Most of the tunneling work with the use of mechanical combine systems is realised with the rocks of medium hardness and abrasiveness [4,5]. Due to the operating experience of roadheaders rotary cutter bits, the main cause for cutter bits malfunction is wearing of the toolholder head accompanied by the breaking out of the hard alloy insert (figure 2). The early wearing of cutter bits in relation to its hard alloy insert leads to incomplete resource consumption of the entire tool. The increased wear resistance of the cutter bit toolholder will lead to the increase of the service life of the rock-destruction tool. The producers use different methods in order to improve the wear resistance of a rock-destruction tool of mining equipment. A considerable amount of research is done in the area of rational design parameters of the cutter bits, the use of sophisticated and hard-wearing materials or the use of special processing techniques.
The special processing techniques include some kinds of heat treatment, in particular, thermomechanical treatment (TMT) combining steel plastic strain at elevated temperature and heat MEACS2016 IOP Publishing IOP Conf. Series: Materials Science and Engineering 177 (2017) 012062 doi:10.1088/1757-899X/177/1/012062 treatment. The formation of the structure of hardened steel during TMT occurs under conditions of increased density of dislocations caused by technological conditions of the hot strain process [3]. Materials processing using this technique causes a significant increase of strength properties of the processed steel [8 -11]. It is known that the increase of material hardness leads to the increase of its wear properties [12]. Consequently, one of the effective ways to improve the wear resistance of rotary cutter bit toolholders is to increase their hardness.
This paper presents an investigation of the influence of different values of strain on the hardness of steel of cutter bit toolholders during thermomechanical treatment.

Experimental methods and materials
The study of the impact of various modes of TMT on material hardness of the road header rockdestruction tool was studied on the basis of steel 30KhGSA, which is the most commonly used material in the production of tangential rotary cutter bit toolholders. The value of strain intensity ε р (effective strain) in the samples surface layer is set as a changing parameter of the TMT mode (other parameters are constant).
Cylinder   (figure 4 b). The plot with а = 3 mm and h = 2 mm was accepted as a working part. Hardness measurements were carried out using universal hardness tester Zwick ZHU 187.5.

Results and Discussion
The results of hardness measurement of samples made of the 30KhGSA steel, subjected to TMT of various strain intensity are presented in table 1.

MEACS2016
IOP Publishing IOP Conf. Series: Materials Science and Engineering 177 (2017) 012062 doi:10.1088/1757-899X/177/1/012062 There are also data on hardness of the head part of cutters toolholders in accordance with GOST R 51047-97 'Cutter bits for roadheaders and shearers. General technical specifications', shown in brackets for comparison.
The table shows that the increase of strain intensity leads to a significant increase of hardness in these samples. In this case, maximum strain ε р = 0.69 conditions of the hardness increase up to 1.2 times compared with the underformed samples.
The curve in figure 5 shows the dependence of hardness variation of the working part of fragments after undergoing TMT, given in terms of HB, on the intensity of plastic strain ε р , to which this area was subjected before hardening. where HB 0 -hardness of the material (steel 30KhGSA) not being subjected to strain before hardening (ε р = 0); ∆HB -hardness increment due to the strain during TMT; A, x -empirical coefficients, constant for a given material (A =73, x =0.73).
The obtained results of experimental researches were used to create a computer model of the TMT process of rotary cutter bit toolholders TMT using the finite-element method of the software package 'DEFORM-3D' (figure 6). The head part of the cutter bit toolholder 'RSh 32-70/16.М1' has the worst wear during cutting of rocks. The change of the direction of the metal flow in the head part of the cutter bit toolholder during forging can increase strain ε р in this part. As a result of the metal flow investigation, we decided to make a bevel in the head part of the workpiece (Figure 7). As a result of simulation of the metal flow process, it was established that the maximum value of strain ε р~0 .7 is achieved when the bevel length is b = 12...14 mm, and the angle is α = 45...55º. It is recommended that the hard alloy insert fixing should be conducted according to the cold fit technology, used when equipping the rotary cutter bits of foreign manufacture. It should be noted that the proposed technology of cutter bits production does not lead to the increase of the cost of their production [3].