Research on a new tool steel cold welding seam

1 Introduction

Since cold working dies and metallurgical spare parts (such as rolls) are subjected to large alternating stress and friction during operation, the surface of the member is required to have high hardness (58-62HRC) and good wear resistance. Better toughness. The component materials are generally made of high quality tool steel and subjected to strict surface heat treatment. However, the harsh working conditions often lead to cracks on the surface of the component, or even partial shedding. These components are expensive to manufacture, and if the whole piece is scrapped, it will cause huge economic losses. Compared with the whole component, the defect size is generally small, and if the repair is performed by an appropriate method, the production cost can be remarkably reduced. Welding surfacing repair is an effective method to extend the service life of components.

At present, the commonly used surfacing welding material is Cr-Mo-W alloy system, which uses mesh carbide and high carbon martensite to improve the hardness and wear resistance of the tool. However, the extremely high hardness and brittleness of the tool surface make it poor weldability. In order to avoid welding cracks, it is necessary to preheat 300-500 °C before welding (sometimes post-weld heat treatment is required), which is for the repair of some large tooling. The process feasibility is extremely poor, which seriously limits the engineering application of the surfacing repair method. It is urgent to study a cold welding repair method with simple process. In order to achieve cold welding of tool steel, in addition to the process measures to reduce the welding stress, it is necessary to improve the toughness and crack resistance of the weld while ensuring the hardness and wear resistance of the weld. The author studied the microstructure of a new type of cold-welded weld by surfacing test and scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction.

2 Surfacing test and inspection

a. Test material: high-carbon Ti-Nb-V-Re-Cr alloy system, through the electrode coating transition C and alloying elements, metal Nb, Ni, ferrotitanium, ferrovanadium, ferrochrome, graphite And Re-Si, Re-Mg alloy; using CaO-CaF2-TiO2 slag system, slag basicity B = 1.3 ~ 1.4; using Ø 3.2mm H08A core, pressing the electrode on TL-25 type electrode coating machine.

b. Test equipment and procedures: The metallographic sample is deposited on the surface of 9Cr2Mo roll by self-made welding rod. The welding current is I=130~150A, the interlayer temperature is less than 35°C; the sample is cut by the grinding wheel cutter under water cooling condition. Microstructure of samples by JXA-840 scanning electron microscope (SEM), electron probe (EPMA), Model 3080E X-ray fluorescence spectrometer, H-80 transmission electron microscope (TEM) and D/max-RC X-ray diffractometer Component analysis. The ZB-28 impact tester was used for the unnotched impact test, and the fracture morphology was observed by SEM. The defects were repaired and inspected on the surface of the Ø 200 mm 9Cr2Mo cold roll.

3 test results and analysis

a. The main components of the weld

The main components of the weld measured by X-ray fluorescence spectrometer are shown in Table 1.

Table 1 Main element content of weld (% by mass, %)

element

C

Ti

Nb

V

Cr

Ni

Mo

content

0.88

0.64

1.18

2.18

1.43

1.3

0.25

It can be seen from Table 1 that the weld is a high carbon multi-alloy system measured by X-ray fluorescence spectrometer, and high carbon is the basis for ensuring the hardness and wear resistance of the weld. Nb, Ti, and V are strong carbide forming elements, which are easy to form MC type carbides. When the content is high, primary carbides can be precipitated from liquid metals. The high hardness of MC carbide (TiC: HV3200, NbC: HV2400, VC: HV2094) is beneficial to improve the hardness and wear resistance of the weld; at the same time, the uniform distribution of primary carbides is beneficial to improve the toughness of the material. Cr and Mo are medium-strong carbide elements, but when they coexist with Ti, Nb, and V, Cr and Mo mainly act as solid solution strengthening, strengthening the matrix, and Mo has a certain effect on refining grains and improving material toughness. The main role of Ni is solid solution strengthening and improvement of toughness. X-ray fluorescence spectrometer also measured the addition of Re element in the weld, in order to use Re oxide to provide nucleation core for carbide and promote the dispersion distribution of carbide.

Figure 1 shows the X-ray diffraction results of the weld specimen. The weld material consists mainly of α-Fe, NbC and TiC. Figure 2 shows the distribution of carbides in the weld. It can be seen that the number of carbides is large and evenly distributed in the form of particles. Electron probe and electron diffraction test results show that the carbides in the weld are mainly NbC, TiC, VC or Nb, Ti, V composite carbides (Nb, Ti, V) C.



figure 1



figure 2

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