Analysis of the Derusting Treatment Process on the Surface of Railway Passenger Car Inspection and Repair Wheel and Axle

0introduction

The existing derusting methods are divided into chemical derusting methods and physical derusting methods.

Chemical rust removal is the use of chemical agents for pickling and rust removal of the workpiece.
In order to facilitate subsequent coating, it will also be accompanied by a series of treatment processes such as phosphating and passivation.

The method for removing rust is relatively uneven, especially for the rust deep in the pit of the base material, which is not easy to remove, has high labor intensity and high labor cost, but has high flexibility and is suitable for small-area operations.

Shot blasting (sand) rust removal is a method of spraying shot or sand to the surface of the workpiece at a high speed with compressed air as the power to obtain a certain degree of cleanliness and roughness on the surface of the workpiece.

This method is currently the most commonly used and most feasible derusting method in the field of rail vehicles, and is widely used in pre-painting operations for newly built and repaired car bodies, bogie components and other products.

Abrasive water jet rust removal is the use of abrasive water with water to spray the surface of the workpiece at high speed, which is a wet sandblasting method for rust removal.

In addition, a water supply system is added to the dry sandblasting, and the equipment and maintenance costs are high.

Laser rust removal is an emerging rust removal method, which uses high-energy pulsed laser to irradiate the surface of the workpiece, and the surface rust layer absorbs laser energy and rapidly rises to generate vaporization, thermal expansion, impact, etc.

In the "Regulations for Assembly, Maintenance and Management of Railway Passenger Wheels and Axes" issued by the China Railway Corporation (Tie Zongyun [2013] No.

191), the visual inspection requirements in Article 3.

Based on the actual production status, this paper selects three different rust removal methods: sandblasting, grinding disc grinding, and angle grinder grinding wheel grinding to verify the surface treatment test and analyze the effect of different treatment methods on the surface roughness, gloss, temperature change, and The impact of the degree of damage to determine the optimal rust removal treatment.

1Test part

1.

1 Material preparation

There are 3 axles, 1 grinder (with 40#, 120#, 240# grinding disc), 1 angle grinder (with 60# grinding wheel), and 1 infrared temperature measuring gun.

1.

2Test content 

For three different rust removal methods: sandblasting, grinding disc polishing, and angle grinder polishing, the wheel and axle surface rust removal treatment verification was carried out.

(1) One axle is selected for sandblasting with quartz sand, and the surface temperature and appearance changes of the axle are recorded during the sandblasting process.

(2) Select 1 axle for grinding disc polishing, divide the axle into 3 sections, and use 40#, 120#, 240# grinding discs for grinding treatment respectively.
Record the surface temperature change and appearance change of the axle during the grinding process.

(3) Select one axle for grinding wheel grinding treatment of angle grinder, and record the change of axle surface temperature and appearance during the grinding process.

2Results and discussion

2.

1 Sandblasting of axles

Before sandblasting, yellow rust can be seen on the exterior surface of the axle, and the overall stain is dark gray; after sandblasting, there is no visible oil stain, no paint coating, no rust adhesion on the surface, and the rough basic metal surface is exposed, and the whole is dull.

Grayish white, as shown in Figure 1, the sandblasting grade reaches the Sa2 level specified in GB/T 8923.

Figure 1 Comparison of the state of the axle before and after sandblasting

2.
2 Grinding treatment of axle grinding discs

Before polishing, yellow rust can be seen on the outer surface of the axle, and the overall stain is dark gray; after polishing with 40#, 120#, 240#, the surface reveals a bright white metal surface with a certain luster, among which, the polished part of the 40# polishing pad is shiny.
The degree of gloss is the strongest, followed by the gloss of the 120# grinding disc, and the gloss of the 240# grinding disc is the weakest.
However, adhesion of rust and dirt can still be seen, so it does not meet the visual inspection requirements of Article 3.
1 of the "Railway Passenger Wheel Axle Assembly, Maintenance and Management Rules" (Tie Zongyun [2013] No.
191).
The appearance effect changes are shown in Figure 2 and Figure 3.
Before polishing, the surface of the axle is about 16°C; after polishing, the surface of the axle is about 17°C; the temperature is basically unchanged.

Figure 2 Comparison of the state of the axle grinding disc before and after grinding treatment

Figure 3 Comparison of the effect of grinding discs with different meshes

2.
3 Grinding treatment of axle angle grinder grinding wheel

As shown in Figure 4, before polishing, yellow rust can be seen on the surface of the axle, and the overall stain is dark gray; after polishing with a 60# angle grinder wheel, there is no visible oil stain on the surface, no paint coating, rust adhesion, and the surface is exposed The shiny metallic luster surface complies with the visual inspection requirements of Article 3.
1 of the "Railway Passenger Wheel and Axle Assembly, Maintenance and Management Rules" (Tie Zongyun [2013] No.
191), but the relatively smooth surface is not conducive to subsequent paint adhesion.
Before polishing, the surface of the axle is about 16°C; after polishing, the surface of the axle is about 19°C; the temperature is basically unchanged.

Figure 4 Comparison of the state before and after the grinding wheel of the axle angle grinder

2.
4 Analysis of mechanism of action

(1) Sandblasting: During the sandblasting and blasting cleaning process, sand particles are sprayed on the surface of the axle to be treated at a high speed to change the outer surface of the axle.
Due to the impact and cutting action of the abrasive on the surface, it is mainly manifested as an impact action, which makes the surface of the wheel shaft obtain a certain degree of cleanliness, and makes the surface of the wheel shaft rough, which is conducive to subsequent paint adhesion.
After sandblasting, the microscopic appearance of the metal surface shows a large number of irregularly shaped impact pits, which play an important role in improving the adhesion between the subsequent coating and the substrate.

(2) Grinding discs: Grinding discs with different roughnesses are rotated under the drive of compressed air to grind, cut and rust the surface.
Compared with sandblasting and angle grinder polishing, the impact and cutting effect of manual grinding on the steel surface is relatively small.
The large area and deep rust of the base material cannot be completely removed, and it is only suitable for partial rust removal.
.
Comparing the polishing of different types of grinding discs, the smaller the number of grinding discs, the rougher the surface of the grinding discs, and the greater the effect on the surface of the steel, which shows that the 40# grinding disc has the strongest degree of rust removal and the strongest gloss; 120# grinding discs The polished area has the second highest degree of rust removal; the polished area of ​​240# grinding disc has the weakest degree of rust removal.

(3) Grinding of angle grinder grinding wheel: Grinding, cutting, rust removal and polishing of metal parts using high-speed rotating grinding wheels.
Due to the cutting structure and high-speed rotation of the grinding wheel, the angle grinder has a strong cutting force on the surface of the steel, and the rust is removed thoroughly, and the surface appears polished, showing a bright metallic luster.
The surface morphology of the angle grinder wheel after grinding is different from the pits produced after sandblasting, and the distribution is uneven striped grooves.

Table 1 The effect and influence of different treatment methods on the surface of the axle

Based on the above-mentioned test results and analysis of the mechanism of action, the following conclusions can be drawn:

(1) The surface roughness of the blasting wheel shaft is the largest, and the rough surface shows diffuse reflection of light.
After blasting, the surface of the wheel shaft is macroscopically dull and gray, which is a normal phenomenon;

(2) Sandblasting has the smallest cutting effect on the surface of the wheel shaft, and the grinding wheel of an angle grinder has the largest cutting effect.
From the perspective of axle protection, sandblasting is the treatment with the least damage to the axles;

(3) The surface temperature of the rear wheel axle is basically unchanged in three different ways, and there is no adverse effect on the axle;

(4) Sandblasting has the most thorough rust removal effect and can obtain a certain degree of roughness on the surface of the axle.
From the point of view of subsequent processes, sandblasting is the most beneficial treatment for subsequent flaw detection and paint construction.

3Applications

In the field of rail vehicles, rust removal mainly uses shot blasting (sand) to remove rust, supplemented by manual and power tools for rust removal.
Generally, shot blasting (sand) cleaning of carbon steel substrates can use metal abrasives such as steel shots and steel wire segments or non-metallic abrasives such as corundum and emery.
After blast cleaning, the steel surface cleanliness level should reach the Sa2 ¹ / specified in GB/T 8923.
1 Level ₂ .
Among them, the blast cleaning of factory-repaired carbon steel car bodies and parts uses steel shots and steel wire section abrasives.
After blast cleaning, the steel surface cleanliness level should reach the Sa2 level specified in GB/T 8923.
1 or the PSa2 level specified in GB/T 8923.
2.

The aluminum alloy substrate is sandblasted with non-metallic abrasives such as brown corundum and emery.
The surface roughness of the aluminum alloy substrate after blast cleaning usually requires Ra (6-20) μm.
For example, domestic railway passenger car bodies with speeds of 160km/h and below, bogie frames of various models, etc.
, are treated with shot blasting.
Car bodies such as high-speed EMUs, intercity EMUs, subways, light rails, and trams made of aluminum alloy materials are sandblasted with brown corundum.
Non-metallic abrasive sandblasting is used to remove rust from other carbon steel welded end underframes, overhaul wheels and axles, and axle boxes.

4 Conclusion

Rust removal is a necessary step before wheel and axle flaw detection and painting.
For the maintenance of the axle, the sandblasting and rust removal treatment has the least damage to the axle, and the rust removal effect is the best, which can not only meet the inspection requirements of the maintenance axle, but also meet the subsequent coating requirements.
At present, shot blasting (sand) rust removal is also the most commonly used and most feasible rust removal method in the field of rail vehicles.
It is widely used in the pre-painting operations of new-built and repaired car bodies, bogie components and other products.

References (omitted)

Authors|  Yue Shujing, Cui Yongxi, Ge Meizhou, etc.

(CRRC Sifang Vehicle Co.
, Ltd.
, Qingdao)