2017 Volume 57 Issue 4 Pages 723-729
The push-off test for measuring the bonding property between surfacing layers and steel base metal was developed after referring to and analysing relevant literature. The push-off test specimens were prepared with surface layers of austenite steel, alloyed steel strengthened by niobium carbide, and high-chromium, hyper-eutectic alloyed, cast iron on a 45 steel base metal by applying a consumable metal electrode arc surfacing welding process with three types of self-shielding, flux-cored, welding wires. The chemical compositions and microstructure of the final product were studied. The bonding strengths of 316, 241, and 60 MPa between three types of surfacing layers and the 45 steel base metal were obtained, after the push-off test. Additionally, the fracture path and fractograph characteristics were observed and determined.
The main processing equipment used in the cement, electric power, and mining and industries, as well as other industries, that have metal components will fail due to wear upon crushing materials. The cyclic high-stress squeezing of hard materials leads to severe fatigue wear due to local fatigue wear. These types of wear result in causing the block to fall off and subsequent shut down for machine repair, which seriously affects the production and results in great economic losses.1,2) For example, in the cement production process, a surfacing method is often used for creating a wear-resistant layer on the steel roller core surface of the squeeze roller of a roller press, which is used for grinding raw material or clinker. For the squeeze roller, in addition to severe surface wear failure, cyclic stress leads to sub-surface metal crack initiation and continues to expand, which will sometimes result in wear-resistant layer flaking and premature failure. To prevent the roller surface failure, the wear resistance and bonding properties of the steel substrate and the working layer should be taken into account. Austenitic steels are typically used as transition metals to improve the bonding strength of the wear-resistant layer and the steel base metal.
On the basis of the relevant literature, this article puts forward a push-off test that can be used to test the bonding properties of the surfacing layer and steel base metal. The self-shielded, flux-cored, arc welding method was employed to prepare different types of surfacing layer metals on 45 steel. The bonding strength of the surfacing layer and 45 steel base metal was obtained by a self-designed push-off test with additional microscopic analysis.
There are several ways to measure the bonding strength of a coating and substrate, such as the transverse stretching method,3,4) the vertical stretching method,5,6) the cantilever bending method,7) and the shear method,8,9) among others. The transverse stretching method is semi-quantitative and used in estimating interfacial adhesion by the interfacial shear stress by applying tensile stress to the coating-matrix system. The vertical stretching method is a method in which a coating or a film coating surface is adhered to an object, which can be easily applied to the load by a certain adhesive (such as epoxy resin). Subsequently, a tensile load is applied to one end of the object to test the bonding strength between the coating and the substrate. The cantilever bending method adopts an acoustic emission technique to judge whether the interface is cracked. The tensile strength of the interfacial bonding is determined according to the geometric dimensions of the cantilever beam and the critical load corresponding to the instantaneous cracking of the interface. The shear method uses a cylindrical specimen with the radius of the uncoated portion close to the radius of a fixed sleeve, in which the protruding coating will be sheared off under load. Reports on testing the bonding strength of surfacing layers and base metal are rare. Zhao10) used tensile specimens to measure the bonding properties of surfacing layers and a steel base metal. The surfacing materials were welded to the two sides of the base material by a Tungsten Inert Gas (TIG) welding method and processed into a plate tensile specimen. Characterization of the bonding properties of surfacing layers and the steel base material measured the tensile strength of the tensile specimens. I. Voutchkov et al.11) designed the “push-off” test, measuring the bonding strength between low carbon steel and a 304 stainless steel, friction stir welding layer, which can be referred to as a “push-off method”, shown in Fig. 1(a). The bonding surface of the surfacing layer and the base material tested by this method is flat; therefore, the bonding strength can be tested by processing the blind hole. However, the depth error of the blind hole machining is large, and the surfacing layer is long strip. The bonding surface is not evenly distributed around the blind hole, when the “push-off test” is carried out, the part near the blind hole is tensile fracture, and the part far from the blind hole is avulsion, which has a great influence on the test results and cannot be used for the non-planar interface between the surfacing layer and the base metal. Referencing this method, we redesigned the push-off method to make it applicable to arc surfacing. First, we processed a through-hole in the base metal, which was followed by surface welding after using a cylindrical inner plug to fill in the hole to make one side of the hole flat. This method avoids the shortcomings concerning the difficulties in processing wear-resistant surfacing metal and the subsequent damage to the surfacing layers caused by the follow-up processing. Meanwhile, surfacing with reference to the hole ensures that the bonding surfaces are evenly distributed around the hole. The bonding surface of the surfacing layer is roughly circular, with the axis of the hole as the centre. The circular bonding surface ensures the uniform force of the specimen during the “push-off” test. The combined area was calculated by AutoCAD software to ensure accuracy; the bonding strength between the surfacing layer and the base metal is high, and in the case of a moderate bonding area, it is appropriate to use a cemented carbide push-off head to transfer the pressure. The project team designed a test method, shown in Fig. 1(b), suitable for testing the bonding strength of the arc surfacing layers and the base metal.
The schematic diagram of push-off test for measuring bonding strength: (a) Test of bond strength between friction stir welding layer and base metal; (b) Test of bond strength between arc surfacing layer and base metal.
As shown in Fig. 1(b), the push-off specimens consisted of a cylindrical inner plug, outer circular base metal, and surfacing layer. The inner plug and outer circular base metal were connected together by the surfacing layer. When the push-off test was conducted, the sample was placed on a pedestal, a push-off head was placed on the inner plug, and then the whole specimen was placed on the universal testing machine. According to the surfacing layer types and bonding area in the base material, the universal testing machine indenter was adjusted to touch the upper surface of the push-off head. The maximum load and loading speed were set before conducting the push-off test. After loading, the thrust passed through the inner plug to the surfacing layer, the bonding surface of the surfacing layer and outer circular base material were subjected to tensile stress, and a fracture occurred when the tensile stress exceeded the bonding strength of the surfacing layer and the outer circular base material. The fracture surface should be near the bonding surface region of both. This push-off specimen can be prepared by applying Shielded Metal Arc Welding (SMAW), gas tungsten arc welding (GTAW), or carbon dioxide gas-shielded welding, among other methods. Herein, Zuo12) used the gas tungsten arc welding method to prepare the push-off specimens. The bonding properties of some surfacing metals and a 45 steel base metal were tested using the push-off test, with sufficient results.
2.2. Materials, Methods, and EquipmentThe base metal used in this study is 45 steel, which corresponds to the standard steel grade C45E4 in international standard ISO683. The composition analysis results of the base metal are shown in Table 1, and its microstructure shows a large quantity of pearlite and small quantities of ferrite. The 45 steel base metal was divided into the following parts: the dimension of the cylindrical inner plug was Φ30 × 15 mm, the dimension of outer circle was Φ60 × 15 mm, and the dimension of centre through-hole was Φ30 mm. At present, the wear-resistant materials widely used in the cement industry are alloy steel and high-chromium alloy cast iron, Austenitic steels are typically used as transition metals to improve the bonding strength of the wear-resistant layer and the steel base metal. The applications of these three materials are typical, so these three types of common self-shielding, flux-cored, welding wires are used in this paper, including austenitic steel, served as the transition layer. Alloyed steel strengthened by niobium carbide and high-chromium, hyper-eutectic alloyed, cast iron were used as the wear-resistant layer; all specifications were Φ2.8 mm. The NBC-630-type inverter welder was used as the welding power source. The following parameters were used: a welding current of 240–280 A, an arc voltage of 26.8–27.5 V, and a wire extension of 15 mm. The air-cooling method was used after welding. Before welding, the inner plug and outer circular base metal were made to match well. Following this step, the components were placed on the chuck of a self-made turntable, in which the reducer was controlled by the inverter of the turntable. The welding speed was maintained at 2 mm/s via adjusting the rotating speed of the turntable. The coincidence area of the arc starting and arc tail should be filled well, as much as possible while welding, to form a circular welding surfacing layer.
| C | Si | Mn | S | P | Cr | Ni | Cu | Fe |
|---|---|---|---|---|---|---|---|---|
| 0.42–0.50 | 0.17–0.37 | 0.50–0.80 | ≤0.035 | ≤0.035 | ≤0.25 | ≤0.25 | ≤0.25 | Bal. |
Four specimens were prepared using each type of flux-cored wire: three of them were used to test bonding strength, and the fourth was used to test the components of the surfacing layer. After grinding the surface of the sample, the optical spectrum analyser was employed to investigate the chemical composition of the upper surface of the surfacing layer. The universal testing machine was used to conduct the push-off test; the maximum thrust was recorded prior to fracture. The photos of typical specimens before and after the push-off test and the fracture surface, which was used to calculate the bonding strength, are shown in Fig. 2.
Typical specimens before and after the push-off test and the fracture surface: (a) surfacing state; (b) after push-off; (c) fracture surface.
The ring-like fracture area in Fig. 2(c) was measured by Auto CAD software, and the bonding strength between the surfacing layer and base metal was calculated by formula (1).
| (1) |
In the formula, F denotes the maximum thrust, and S denotes the fracture surface area.
After the surfacing layer was separated from the sample, the wire cutting method was used to prepare the metallurgical fracture analysis sample using an Axio-Scope. An A1 microscope and JXA-840 scanning electron microscopy were used to analyse the microstructure of surfacing layer, the fracture path, and the fractograph characteristics.
Surface composition analysis results of the three types of flux-cored surfacing layers are shown in Table 2. The surfacing layer microstructure was observed and analysed by metallurgical microscope.
| Flux-cored welding wire type and number | The main component of surfacing layer | |||||||
|---|---|---|---|---|---|---|---|---|
| Element | C | Si | Mn | Cr | Nb | Ni | Mo | Fe |
| Austenitic steel | 0.09 | 0.83 | 5.32 | 14.68 | -- | 6.62 | -- | Bal. |
| Alloyed steel strengthened by niobium carbide | 1.43 | 0.55 | 0.81 | 5.22 | 5.85 | 0.10 | 1.30 | Bal. |
| High-chromium hyper-eutectic alloyed cast iron | 4.46 | 0.77 | 0.76 | 23.54 | -- | 0.11 | -- | Bal. |
Table 2 and Fig. 3 show that the surfacing layer corresponding to the austenitic steel wire contained a high amount of Cr and other austenitizing elements, such as Mn and Ni. Shown in Fig. 3(a), the microstructure of the surfacing layer was dendritic, single-phase austenite. By observing the microstructure at the interface between the bottom of the surfacing layer and the base metal heat affected zone (HAZ), the united austenite dendrites grown on the pearlite bonded well with the base metal in the HAZ high-temperature region, as shown in Fig. 3(b). The niobium-alloyed surfacing layer contained a high amount of C, Cr, Nb, and Mo elements. The microstructure, which was martensite, retained austenite and dispersed the NbC particles, with few sleek massive austenite (Fig. 4(a)). As shown in Fig. 4(b), a layer of needle-like martensite was found on the interface of the surfacing layer metal and base metal (the fusion zone); no sleek massive austenite was found at the bottom of the surfacing layer, and this zone contained small dispersed small NbC particles. The high-chromium, hyper-eutectic alloyed, cast iron surfacing layer contained a large amount of Cr and C; the microstructure was a mixed structure, which consisted of strip and hexagonal primary carbides and eutectic carbides, as shown in Fig. 5(a). The fusion zone was composed of a pearlite layer, a discontinuous (martensite + retained austenite) thin layer, and austenite layer. The bottom of the surfacing layer, which was closed to the fusion zone was composed of a large amount of eutectic and a small amount of austenite dendrites, as shown in Fig. 5(b). In addition, in the welding conditions of this article, the high-temperature region of the 45 steel material HAZ was composed of the coarse Widmanstatten structure.
Microstructure of surfacing layer: (a) (b) austenitic steel.
Microstructure of surfacing layer: (a) (b) alloyed steel strengthened by niobium carbide.
Microstructure of surfacing layer: (a) (b) high-chromium hyper-eutectic alloyed cast iron.
The bonding strength of the three types of surfacing layers and 45 steel was measured by the push-off test in Fig. 1(b), and the results are shown in Table 3 and Fig. 6. The bonding strengths of the three types surfacing layers and 45 steel were 316 MPa, 241 MPa, and 60 MPa. It can be seen in Fig. 6, the bonding failure between three different surfacing layers and 45 steel base metal is brittle fracture.
| Type of surfacing layer metal | Maximum push-off force/KN | Bonding area/mm2 | Bonding strength/MPa | Average/MPa |
|---|---|---|---|---|
| Austenite steel | 189 | 655 | 288 | 316 |
| 187 | 586 | 318 | ||
| 182 | 533 | 341 | ||
| Alloyed steel strengthened by niobium carbide | 95 | 515 | 184 | 241 |
| 84 | 346 | 242 | ||
| 71 | 239 | 296 | ||
| High-chromium hyper-eutectic alloyed cast iron | 41 | 476 | 86 | 60 |
| 28 | 476 | 58 | ||
| 17 | 463 | 36 |
Force-displacement curve of bonding strength between surfacing layer and base metal.
The push-off test primarily measures the bonding strength of the surfacing metal interface region and base metal. There is a large composition difference between the surfacing metal and base metal; they have completely different microstructures. These differences make the fusion zone a weak area among the specimens. The fracture path was located in the fusion zone under normal circumstances. The bonding strength was directly related to the composition and microstructure of the fusion zone and the nearby zone. Therefore, the fracture path and its microstructure characteristics were analysed to determine the main factor that impacted the bonding strength of the surfacing metal and 45 steel base metal, and the fractograph analysis was carried out at the same time.
The fracture path and fractograph characteristics of the austenitic steel surfacing layer and 45 steel base metal is shown in Fig. 7. The strength and ductility, which was excellent due to the presence of the surfacing layer, was austenitic steel, as shown in Fig. 7(a). The main crack started in the HAZ parts of the base metal side, then tore the pearlite and ferrite, which had a lower intensity along the high-temperature zone of the HAZ. This crack expanded along the high-temperature area of the base metal HAZ, and the HAZ metal fracture finally formed, whereas the secondary cracks went into the inner HAZ. HAZ coarse grains could be observed from the macro-fracture morphology in Fig. 7(b), which was a cleavage fracture in the micro-fracture morphology, thus demonstrating there is a brittle fracture in HAZ, as shown in Fig. 7(c).
Fracture path and fractograph characteristics of the austenitic steel surfacing layer and 45 steel base metal: (a) fracture path; (b) macro-fracture morphology; (c) micro-fracture morphology.
Figure 8 shows the fracture path and fractograph characteristics of alloyed steel strengthened by the niobium carbide surfacing layer and the 45 steel metal. As shown in Fig. 8(a), the crack started in the root of the surfacing layer, extended to the bottom of the surfacing layer, and then, secondary cracks appeared. Since some austenitic was contained in the surfacing layer, it possessed a higher strength and better toughness than cast iron surfacing layer. The crack extended into the HAZ of the base metal until the outside of the specimen expanded. Finally, cracks occurred in both the left side of base material and in the right side of surfacing layer, resulting in a mixed fracture. The macro-fracture and micro-fracture morphologies of the mixed fracture characteristics of the HAZ and surfacing layer are shown in Figs. 8(b) and 8(c), respectively. The upper part shows the fine grain fracture surface of the surfacing metal, where the tears and cleavage fractures coexist; the lower part shows the HAZ cleavage fracture.
Fracture path and fractograph characteristics of alloyed steel strengthened by niobium carbide surfacing layer and 45 steel base metal: (a) fracture path; (b) macro-fracture morphology; (c) micro-fracture morphology.
Figure 9 exhibits the fracture path and fractograph characteristics of the high-chromium, hyper-eutectic alloyed, cast iron surfacing layer and 45 steel base metal. As shown in Figs. 9(a) and 9(b), the crack started in the root of the surfacing layer, extended along the interface of the surfacing layer and base metal at first, then expanded into the surfacing layer bottom’s hypoeutectic region. After the crack branched, the main crack came back to the fusion zone and extended along the interface of the austenite layer and hypoeutectic compositions until reaching the outer surface of the specimen. The secondary crack, which extended into the internal surfacing layer, had a longer extended distance, but no cracks passed through it. As shown in the microstructure characteristics in Fig. 9(c), we can know that in the hypoeutectic region of the surfacing bottom layer, cracks mainly extended along the eutectic compositions, partially passing through austenite dendrites. In the hypereutectic region of the upper part of surfacing layer, the crack mainly extended along the boundary of the primary carbides and eutectic compositions, and primary carbide fragmentation was observed (shown in Fig. 9(d)). Figure 9(e) shows a macroscopic fractograph of the sample, which shows the characteristics of the brittle fracture. Figure 9(f) shows the characteristics of the microscopic fracture morphology, which comprised a cleavage brittle fracture with hypoeutectic features.
Fracture path and fractograph characteristics of the high-chromium, hyper-eutectic alloyed, cast iron surfacing layer and 45 steel base metal: (a) fracture path; (b) crack propagation path; (c) crack propagation path of surfacing layer bottom; (d) crack propagation path of surfacing layer upper; (e) macro-fracture morphology; (f) micro-fracture morphology.
In conclusion, with cracking of austenite steel and fracture growth from the grain boundary, the fracture morphology can be characterized by cleavage fracture and a tiny flat, fan-shaped fracture. The fracture of alloyed steel strengthened by niobium carbide shows a rough steppe morphology with secondary cracks; both cleavage fracture and tear fracture exist in the surface of the fracture. Feathery figures were observed in the fracture of high-chromium, hyper-eutectic alloyed, cast iron; they appear as a river pattern of predominant cleavage and the crack growth along the grain boundaries. All the failure between these three surfacing layers with different microstructure and base metal is due to the brittle cleavage fracture.
From the analysis results of the bonding strength of the surfacing metal and base metal, the fracture path, and fractograph, it was feasible to test the bonding properties of the arc surfacing layer and the base metal. The bonding strength of the surfacing metal and steel base metal had a direct relationship with the microstructure of the fusion zone metal of the bottom of the surfacing layer and HAZ.
The aim of this study was to research the bonding properties of wearing materials in squeezing rollers used in cement production. The push-off test was redesigned to test the bonding properties of the ace-surfacing specimens; the microstructure and mechanical properties of the wearing material were analysed herein, and the following conclusions could be drawn:
(1) A push-off test to measure the bonding property between surfacing layers and steel base metal was put forward.
(2) The push-off test specimens were prepared with surfacing layers of austenite steel, alloyed steel strengthened by niobium carbide, and high-chromium, hyper-eutectic alloyed, cast iron on a 45 steel base metal by applying self-shielding, flux-cored, welding wires and a consumable metal electrode arc surfacing welding process. The bonding strengths of the three types of surfacing layers and 45 steel base metal were 316 MPa, 241 MPa, and 60 MPa.
(3) Austenitic steel push-off specimens fractured in the coarse grain region of the base metal HAZ showed cleavage fractures. The fracture region of alloyed steel strengthened by the niobium carbide push-off specimen contained both fusion zone metal of the bottom of the surfacing layer and HAZ of the base metal, forming a mixed fracture. The hypereutectic, high chromium alloy, cast iron push-off specimen fractured at the bottom of the surfacing layer, showing brittle fracture characteristics, and a long secondary crack was observed.
