CN107214406A - Control the process of " white block " tissue in bainite rail flash welding joint - Google Patents

Abstract

The invention belongs to the technical field of rail welding, and specifically discloses a process method for controlling the "white block" structure in a flash welding joint of a bainite rail, aiming at solving the problem of the flash welding joint formed by the welding of the existing bainite rail welding technology Problems with a high number and size of "white patches" in the microstructure. During the welding process of the bainite rail by means of rail flash butt welding equipment, the process method sequentially controls the flashing stage, the pulsation stage, the accelerated burning stage, the upsetting stage and the pressure holding stage. By using this process to control the welding process of bainitic rails, the number of "white block" structures near the weld can be minimized, the size of the "white block" structures can be reduced, and its appearance can be improved, thereby Stable welding quality, ensure the mechanical properties of welded joints, improve the service performance of bainite rail welded joints, and ensure the safety of railway operation.

Description

Process method for controlling "white block" structure in flash welded joint of bainite rail

technical field

The invention belongs to the technical field of rail welding, and in particular relates to a process method for controlling the "white block" structure in a bainite rail flash welding joint.

Background technique

Bainitic steel rail has been a research hotspot in the world in recent decades. It is expected to replace the traditional pearlitic steel rail due to its high toughness, wear resistance and long service life. It is widely used in railway turnout components and small rails for heavy-duty lines. Radius curve segment. At this stage, rail seamlessness has become an inevitable trend. As an important process in the seamless link of rails, the quality of rail welding is directly related to the service life of railway lines and even the safety of traffic. During the rail service process, due to the influence of welding quality and the complexity of the actual operating conditions of the line, most of the fractures of the welded long rails occur on the welded joints, so the welded joints have become the weak link of the seamless line.

The flash welded joint of bainite rail has a complex microstructure, and there are white microdomains and banded structures near the weld. The white microdomains, commonly known as "white lumps" are the result of the enrichment of grain boundary elements in the semi-melted zone of welding. , the reasons for its bright white appearance are element enrichment and tissue differences. "White block" is essentially a high-alloy region of the grain boundary with enriched elements. It is a martensitic structure produced under the condition of welding air-cooling due to the melting or liquefaction of the grain boundary of the high-alloy region under the high temperature condition of rail welding. Corroded by the etchant, it appears white. Banded segregation will affect the impact toughness of the joint, and "white blocks" are mostly generated in the banded structure. The main cause of flash welding joint damage is that microcracks are easily generated on the "white block", and the cracks expand along the direction of the joint band to form rail head nucleation, which is the main cause of rail fracture.

In terms of weight percentage, the chemical composition content of bainite rail is generally: C 0.18-0.30%, Si 0.8-1.8%, Mn 1.5-2.5%, Cr 0.50-1.60%, Mo 0.20-0.50%, the balance is Fe and impurities. Using heat treatment technology, based on the principle of fine-grain strengthening, the bainite rail produced and manufactured based on the welding heat cycle will cause the hardened layer in the weld area to disappear and a wider low-hardness zone will appear, resulting in the hardness of the weld and the heat-affected zone. Much lower than the base metal of the rail. Moreover, using the existing bainite rail welding technology, the microstructure of the flash welded joint formed by welding has a large number of "white blocks" and a large size, which will easily lead to crack expansion of the welded joint and shorten the service life of the rail.

Contents of the invention

The invention provides a process method for controlling the "white block" structure in the flash welding joint of bainite rail, aiming at solving the "white block" in the microstructure of the flash welding joint formed by the welding of the existing bainite rail welding technology Problems with large numbers and large sizes.

The technical scheme adopted by the present invention to solve the technical problem is: a process method for controlling the "white block" structure in the flash welding joint of the bainite rail. The flash flat stage, pulsation stage, accelerated sintering stage, upsetting stage and pressure holding stage are controlled as follows:

Control the welding heat input in the flashing stage to be 1.4~2.3MJ, and the duration is 19~23s;

Control the welding heat input in the pulsation stage to be 4.0-5.1MJ, the rail consumption to be 2.2-3.1mm, and the duration to be 52-62s;

Control the welding heat input in the accelerated burning stage below 0.7MJ, the rail consumption is 7.5-8.8mm, and the duration is 20-24.5s;

Control the welding heat input in the upsetting stage below 0.1MJ, the upsetting amount is 14.0-15.5mm, the live upsetting time is 0.3-0.5s, and the duration is 7.4-8.1s;

Control the holding pressure in the holding stage at 26.0-28.0t, and the duration is 10-14s;

Moreover, the total welding heat input is controlled at 7.0-8.5MJ, and the total duration of welding is 105-120s.

Further, the bainitic steel rail is a 60kg/m steel rail.

Further, the method for welding the welded joint of the bainite rail includes mobile flash welding and/or stationary flash welding.

Further, the rail flash butt welding equipment is a mobile flash welding machine.

The beneficial effects of the present invention are: through the process method, during the welding process of bainite rails, the flash flat stage, pulsation stage, accelerated sintering stage, upsetting stage and pressure holding stage are sequentially controlled, and the total amount of welding heat input Controlling the total duration of welding can effectively control the number of "white lumps" near the weld seam of bainitic rail flash welding joints, and within the control range of this process method, the "white lumps" near the weld seam can be controlled The number of "tissues is reduced to a minimum, and the size of the "white block" tissue is reduced, and its appearance is improved, so that the welding quality is stable, the mechanical properties of the welded joints are guaranteed, and the service performance of the welded joints of the bainite rail is improved to ensure that safety of railway operations.

Description of drawings

Fig. 1 is the metallographic micrograph at the bottom corner of the bainite rail flash welded joint rail in Example 1 of the present invention;

Fig. 2 is a metallographic micrograph at the bottom corner of the welded joint of the bainite rail in Example 1 of the present invention under a larger magnification;

Fig. 3 is a metallographic micrograph at the bottom corner of the flash welded joint of the bainite rail in Example 2 of the present invention;

Fig. 4 is a metallographic micrograph at the bottom corner of the welded joint of the bainite rail in Example 2 of the present invention under the same magnification as Fig. 2;

Fig. 5 is a metallographic micrograph at the bottom corner of the bainite rail flash welded joint in Example 3 of the present invention;

Fig. 6 is a metallographic micrograph at the bottom corner of the welded joint of the bainitic rail in Example 3 of the present invention under the same magnification as Fig. 2;

Fig. 7 is the metallographic micrograph at the bottom corner of the welded joint of the bainite rail in Comparative Example 1;

Fig. 8 is a metallographic image at the bottom corner of the welded joint of the bainite rail in Comparative Example 1 under the same magnification as Fig. 2;

Fig. 9 is a metallographic micrograph at the bottom corner of the welded joint of the bainite rail in Comparative Example 2;

Fig. 10 is a metallographic image at the bottom corner of the welded joint of the bainite rail in Comparative Example 2 under the same magnification as Fig. 2;

Marked in the figure: 1-weld, 2-"white block" organization.

detailed description

The present invention will be further described below in conjunction with accompanying drawing.

There are generally multiple welded joints on the bainite rail, and the welding method of the welded joints of the bainite rail includes mobile flash welding and/or fixed flash welding, that is, all welded joints can be mobile flash welding or Fixed flash welding can also be that part of the welded joints adopts mobile flash welding, and the other part of welded joints adopts fixed flash welding. In the present invention, the "welded joint" is a region obtained after welding, including the weld seam 1 and/or the heat-affected zone, with a length ranging from 80 to 120 mm, and the center of this region is the rail weld seam.

Before welding, the following preparations are usually required:

1. Grind the waist of the bainite rail. The length of the grinding part is approximately equal to the length of the clamping electrode in the welding equipment, generally not less than 400mm.

2. Grind the end face of the welded joint of the bainite rail in full section.

The inventors found that the "white block" structure is produced during the rail welding process, and the effective control of the total heat input during the welding process is the main factor determining the number of "white block" in the microstructure of the welded joint. If it cannot be effectively controlled The amount of "white block" tissue in the welded joint will affect the smoothness of the line and even the driving safety.

In order to make up for the deficiencies of the existing bainite rail welding technology, the present invention provides a process for controlling the "white block" structure in the flash welded joint of bainite rail. During the rail welding process, the flash flat stage, pulsation stage, accelerated sintering stage, upsetting stage and pressure holding stage are controlled in sequence as follows:

Control the welding heat input in the flashing stage to be 1.4~2.3MJ, and the duration is 19~23s;

Control the welding heat input in the pulsation stage to be 4.0-5.1MJ, the rail consumption to be 2.2-3.1mm, and the duration to be 52-62s;

Control the welding heat input in the accelerated burning stage below 0.7MJ, the rail consumption is 7.5-8.8mm, and the duration is 20-24.5s;

Control the welding heat input in the upsetting stage below 0.1MJ, the upsetting amount is 14.0-15.5mm, the live upsetting time is 0.3-0.5s, and the duration is 7.4-8.1s;

Control the holding pressure in the holding stage at 26.0-28.0t, and the duration is 10-14s;

Moreover, the total welding heat input is controlled at 7.0-8.5MJ, and the total duration of welding is 105-120s.

Among them, in the welding process using the mobile flash welding method, the mobile flash welding machine is generally selected as the rail flash butt welding equipment. The method of the present invention is preferably used to control the "white block" structure in the flash welding joint of the 60kg/m bainite rail. The flash welding equipment used, the welding process of the equipment is divided into 11 stages in total, of which stage 1 is the flash flat stage, stages 2, 3, and 4 are pulsating stages, and stages 5, 6, 7, 8, and 9 are accelerated burning stage, the 10th stage is the upsetting stage, and the 11th stage is the pressure holding stage.

The flashing stage is to use the flashing liquid lintel blasting function to burn off the dirt and local unevenness on the end face of the rail, improve the microcosmic parallelism of the two end faces to be welded, and create conditions for the subsequent flashing and heating to be uniform.

The pulsation stage is the stage in which the flash welder performs pulsation flash welding. The basic principle of pulsation flash welding is to superimpose a vibration on the feeding process of the welding chuck, so that the contact and separation of the end face of the weldment are alternately carried out in the flash stage. . Electric heating is applied during contact, and when the contact point on both ends is still in solid state, it is pulled off at a high speed to separate the end faces, and this is repeated until the flashing ends.

In the accelerated burning stage, the flash burning speed of this stage is not affected by the welding current, and is gradually accelerated according to the established procedure. The rail is continuously accelerated and fired without stopping or retreating, forming a fierce flash between the end faces of the rail, creating a good protective atmosphere, which can reduce the influence of the controlled oxidation of the end face of the rail and create favorable conditions for upsetting.

In the upsetting stage, after the flashing is finished, the jaws clamp and drive the rail to approach and close the two high-temperature end faces at an appropriate speed, and apply appropriate upsetting pressure to the rail joint to make the liquid metal and possible defects flow out of the rail. Extrude in the gap between the end faces to ensure sufficient plastic deformation at the joint to form a common grain and obtain a firm butt joint.

The pressure-holding stage, that is, the pressure-holding and pushing-out stage, is a process in which a tool that is the same as the cross-sectional profile of the rail is advanced longitudinally along the surface of the rail to remove the welding protrusion.

Adopting the process method of the present invention to control can effectively reduce the amount of "white block" tissue 2 near the welded joint of bainite rail, and minimize the amount of "white block" structure 2 near the weld 1, thereby avoiding defects in the welding area The resulting welded joint cracks expand, prolong the service life of the rail, and ensure the safety of the railway operation.

Example 1

Through the method of the present invention, during the welding process of the bainite rail by the rail flash butt welding equipment, the following control is performed on the flash flat stage, the pulsation stage, the accelerated burning stage, the upsetting stage and the pressure holding stage in sequence:

Control the welding heat input in the flashing stage to 2.2MJ, and the duration is 22s;

The welding heat input in the control pulsation stage is 4.9MJ, the rail consumption is 2.5mm, and the duration is 53.9s;

Control the welding heat input in the accelerated burning stage to 0.6MJ, the rail consumption to 8.2mm, and the duration to 20.5s;

Control the welding heat input in the upsetting stage to 0.1MJ, the upsetting amount to 14.1mm, the live upsetting time to 0.3s, and the duration to 8.1s;

The holding pressure of the control holding stage is 27.5T, and the duration is 12.7s;

The total welding heat input is controlled to be 7.8MJ, and the total welding duration is 108s.

Example 2

Through the method of the present invention, during the welding process of the bainite rail by the rail flash butt welding equipment, the following control is performed on the flash flat stage, the pulsation stage, the accelerated burning stage, the upsetting stage and the pressure holding stage in sequence:

Control the welding heat input in the flashing stage to 2.3MJ, and the duration is 23s;

The welding heat input in the control pulsation stage is 5.1MJ, the rail consumption is 3.0mm, and the duration is 57.9s;

Control the welding heat input in the accelerated burning stage to 0.7MJ, the rail consumption to 8.5mm, and the duration to 20.5s;

Control the welding heat input in the upsetting stage to 0.1MJ, the upsetting amount to 15.3mm, the live upsetting time to 0.5s, and the duration to 7.8s;

Control the holding pressure in the holding stage to 26.7T, and the duration is 11s;

The total welding heat input is controlled to be 8.2MJ, and the total duration of welding is 120s.

Example 3

Through the method of the present invention, during the welding process of the bainite rail by the rail flash butt welding equipment, the following control is performed on the flash flat stage, the pulsation stage, the accelerated burning stage, the upsetting stage and the pressure holding stage in sequence:

Control the welding heat input in the flashing stage to 2.3MJ, and the duration is 22.5s;

The welding heat input in the control pulsation stage is 5.1MJ, the rail consumption is 2.8mm, and the duration is 52s;

Control the welding heat input in the accelerated burning stage to 0.5MJ, the rail consumption to 7.9mm, and the duration to 23.5s;

Control the welding heat input in the upsetting stage to 0.1MJ, the upsetting amount to 14.9mm, the live upsetting time to 0.4s, and the duration to 8.1s;

The holding pressure of the control holding stage is 27.5T, and the duration is 12.9s;

The total welding heat input is controlled to be 8.0MJ, and the total welding duration is 119s.

As shown in Fig. 1, Fig. 3 and Fig. 5, after being controlled by the method of the present invention, the number of "white block" structures 2 around the weld seam 1 is extremely small and the area is small. Observing the metallographic photos of the flash welded joint of the bainite rail, the weld 1 is a white and bright decarburized layer, and the structure is ferrite and bainite. In the area near the weld, there are some irregularly distributed white micro-domains, which are "white block" structures2.

As shown in Fig. 2, Fig. 4, and Fig. 6, under a larger magnification, the metallographic micrograph at the bottom corner of the welded joint of the bainite rail in the embodiment of the present invention shows that the inside of the flash welded joint of the bainite rail The "white block" structure 2 is hardly visible, and the "white block" structure 2 with a diameter of only about 20 μm appears in the middle of the bainite structure.

Comparative example 1

In the process of welding bainitic rails by using the rail flash butt welding equipment, the technical means similar to and different from the process method of the present invention are used to sequentially control the flashing stage, the pulsation stage, the accelerated burning stage, the upsetting stage and the pressure holding stage as follows:

Control the welding heat input in the flashing stage to 2.1MJ, and the duration is 21.9s;

The welding heat input in the control pulsation stage is 6.6MJ, the rail consumption is 8.0mm, and the duration is 72.3s;

Control the welding heat input in the accelerated burning stage to 0.4MJ, the rail consumption to 10.9mm, and the duration to 20.5s;

Control the welding heat input in the upsetting stage to 0.1MJ, the upsetting amount to 14.4mm, the live upsetting time to 0.2s, and the duration to 7.3s;

The holding pressure of the control holding stage is 27.0T, and the duration is 12.8s;

The total welding heat input is controlled to be 9.2MJ, and the total welding duration is 126s.

As shown in Figure 7, this picture is a metallographic photo of a bainite rail flash welded joint with a total welding heat input of 9.2MJ. Similarly, the white bright band in the middle is weld 1, and the structure is ferrite and bainite Organization, heat-affected zone organization is mostly bainite organization.

As shown in Figure 8, the metallographic micrograph at the bottom corner of the bainitic rail welded joint in Comparative Example 1 under the same magnification shows that there are more "white blocks" in the unit microscopic area of the bainitic rail flash welded joint Tissue 2, and the "white patch" tissue 2 is large in size and irregular in shape, and the largest "white patch" tissue 2 is about 80 μm in length.

Comparative example 2

In the process of welding bainitic rails by using the rail flash butt welding equipment, the technical means similar to and different from the process method of the present invention are used to sequentially control the flashing stage, the pulsation stage, the accelerated burning stage, the upsetting stage and the pressure holding stage as follows:

Control the welding heat input in the flashing stage to 2.2MJ, and the duration is 21.8s;

The welding heat input in the control pulsation stage is 3.3MJ, the rail consumption is 1.9mm, and the duration is 35.0s;

Control the welding heat input in the accelerated burning stage to 1.1MJ, the rail consumption to 11.2mm, and the duration to 29.5s;

Control the welding heat input in the upsetting stage to 0.0MJ, the upsetting amount to 13.4mm, the live upsetting time to 0.2s, and the duration to 6.3s;

The holding pressure of the control holding stage is 29.0T, and the duration is 12.9s;

The total welding heat input is controlled to be 6.6MJ, and the total welding duration is 99s.

As shown in Figure 9, it is a metallographic photo of the flash welded joint of bainite rail with a welding heat input of 6.6MJ. It can be seen that under the condition of this welding heat input, the number of "white block" structures 2 is more and the size is larger ;

As shown in Figure 10, the metallographic micrograph at the bottom corner of the welded joint of the bainite rail in Comparative Example 2 under the same magnification shows that there is a clear boundary between the "white block" structure 2 and the bainite structure, and is relatively Hard to corrode.

Conclusion: Through the comparative analysis of the metallographic photos of the above welding heat inputs, it can be known that the number of "white block" structures 2 in the structure of the bainite rail flash welded joints in the range of welding heat input controlled by the design of the present invention is significantly reduced, And the size is small. "White block" is the main welding defect in the welded joint of bainite rail. Reducing its number plays a vital role in ensuring the quality of the flash welded joint of bainite rail.

Claims (4)

1. The technological method of controlling the "white block" organization in the flash welding joint of bainite rail, which is characterized in that: during the welding process of bainite rail using rail flash butt welding equipment, the flash flat stage, pulsation stage, and accelerated burning are sequentially performed stage, upsetting stage and holding stage are controlled as follows: Control the welding heat input in the flashing stage to be 1.4~2.3MJ, and the duration is 19~23s; Control the welding heat input in the pulsation stage to be 4.0-5.1MJ, the rail consumption to be 2.2-3.1mm, and the duration to be 52-62s; Control the welding heat input in the accelerated burning stage below 0.7MJ, the rail consumption is 7.5-8.8mm, and the duration is 20-24.5s; Control the welding heat input in the upsetting stage below 0.1MJ, the upsetting amount is 14.0-15.5mm, the live upsetting time is 0.3-0.5s, and the duration is 7.4-8.1s; Control the holding pressure in the holding stage at 26.0-28.0t, and the duration is 10-14s; Moreover, the total welding heat input is controlled at 7.0-8.5MJ, and the total duration of welding is 105-120s. 2. The process for controlling the "white block" structure in the flash welded joint of a bainite rail as claimed in claim 1, characterized in that: the bainite rail is a 60kg/m rail. 3. The process method for controlling the "white block" structure in the flash welded joint of bainite rail as claimed in claim 1 or 2, characterized in that: the method of welding the welded joint of the bainite rail comprises mobile flash welding and/or stationary flash welding. 4. The process method for controlling the "white block" structure in the flash welding joint of bainite rail as claimed in claim 1, characterized in that: the rail flash butt welding equipment is a mobile flash welding machine.