JPS61104021A - Method for preventing age cracking - Google Patents

Abstract

PURPOSE:To prevent semipermanently the age cracking of a steel which is liable to produce residual stress or to cause work hardening by grinding by shot-blasting the steel as a high-alloy steel after grinding. CONSTITUTION:The ingot of heat resistant austenitic steel which is liable to produce the residual stress or to cause the work hardening in the surface layer by the grinding is cogged to produce a steel billet. This billet is ground, and after the lapse of about 3hr, the ground part is shot-blasted with the steel shot of about 1mm grain diameter at about 60m/sec initial rate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing age cracking that occurs after grinding in high alloy steel such as austenitic stainless steel mesh and heat-resistant steel. Residual stress and work hardening occur in the surface layer of the high alloy steel and the like due to grinding, and if left for about one week to one month after processing, minute cracks (age cracks) will occur on the machined surface.

In particular, it is thought that eave corrosion cracking occurs due to grinding of slab-rolled steel pieces. In other words, the tensile residual stress generated on the surface of the steel piece due to strong grinding during flaw removal, and the surface being activated by grinding, making it more susceptible to corrosion, and corrosion due to atmospheric humidity, sea salt particles, etc. are involved. do,
Microscopic cracks occur on the ground surface.

Conventionally, as a method of preventing the above-mentioned age cracking, in order to remove either stress or corrosion that causes cracking, for example, anti-rust oil is applied to prevent corrosion on the surface activated by grinding. However, this method had only a temporary effect and could not be used as a drastic countermeasure, and the best solution was to remove the stress remaining on the surface of the steel piece.

Conventionally, when residual stress and work hardening occur on the surface due to cold working, etc., annealing treatment is generally performed to remove these stresses, etc. It is difficult to perform annealing treatment after grinding on long materials with large cross-sections, from both economical and productivity standpoints, and it is necessary to develop a simple method to remove residual stress after grinding. It was requested.

The present invention has been made in view of the above-mentioned drawbacks of the prior art. Since stress exists only in the surface layer,
The present inventors applied compressive stress commensurate with the tensile stress to only the surface where residual stress exists, without removing it by heat treatment such as annealing. In the present invention, we have selected shot blasting as a method to remove residual stress on the surface of steel materials with a simple structure, without being restricted by the shape of the steel material, and at a lower cost. This is what I did. In other words, the present invention removes the residual stress itself, and does not temporarily prevent cracking by applying anti-corrosion oil to the grinding surface, which is the case in the past. There is nothing to do.

In other words, the present invention provides for a short period of time after grinding a steel material such as high-alloy steel that tends to cause residual stress or work hardening during grinding, depending on the environment in which the steel piece is placed, the residual stress, or the type of steel. However, shot blasting is preferably performed after soaking for one week.The present invention is an effective method for preventing aging cracking of large steel materials such as steel ingots and slabs, and has a simple structure and is suitable for This is a cheaper treatment method that is not limited by the shape of the treated material.

The present invention will be explained below based on one embodiment.

Austenitic heat-resistant steel was used as a test material as a steel material that tends to cause residual stress and work hardening in its surface layer during grinding, and test materials 1 and 6 were made into 95 square steel slabs by blooming a steel ingot. was manufactured.

Since there were some flaws in the surface layer of the slab-rolled steel billet, the flaws were removed using a grinder.

Then, the residual stress existing on the surface of the steel piece that was ground by the grinder was measured and the result was 140 kg /
A tensile stress of about m rd was present.

As a method for removing the residual stress from a steel piece that has the above-mentioned residual stress, the method of the present invention reduces the grain size to
Shot blasting was applied to the ground portion using a No. 11 steel ball at an initial velocity of 60 m/sec. As a result of measuring the residual stress after shot blasting the ground part, it was confirmed that the residual stress existing in the core part was a compressive stress of 30 kg/mrrl.

In addition, as a comparison material, a steel billet B made of the same steel type, subjected to blooming rolling under the same conditions, and still being subjected to grinding under the same conditions,
Immediately after grinding, a steel piece C was prepared which was coated with antirust oil.

Then, steel billet A subjected to shot blasting according to the method of the present invention
Table 1 shows the steel billet B as-ground as a comparative method and the order status for effective cracking. In addition, in Table 1, the O mark indicates the hour.
No aging cracking occurs, and x marks indicate aging cracking.

As is clear from Table 1 in Table 1 of Table 1, micro cracks (aging cracks) occurred in steel billet B, which was ground using the comparative method and was not subjected to any treatment, one month after grinding. Also, while the steel billet C coated with anti-corrosion oil using the comparative method also developed age cracking after 1111 months of grinding, the steel billet A produced using the method of the present invention did not develop age cracking even after 2 months of grinding. was not recognized.

In the above examples, austenitic hot steel was used as the test material, but aging cracking can be prevented in high alloy steels such as 5OS304 by applying shot blasting after grinding, as in the above examples. Ta.

As mentioned above, the present invention provides a steel material that is prone to residual stress or work hardening during grinding, and is designed to reduce stress after grinding. +7 blasting is applied, and it is a semi-permanent age cracking prevention method that has a simple structure and is not restricted by the shape of the treated material, and has an extremely large contribution to industry.

Claims (1)

[Claims] A method for preventing age cracking, which involves applying shot blasting after grinding to steel materials that tend to produce residual stress or work hardening during grinding, such as high-alloy steel.