GB2072703A

GB2072703A - Stress-relief annealing of ferrous metals

Info

Publication number: GB2072703A
Application number: GB8108860A
Authority: GB
Original assignee: Usui Kokusai Sangyo Kaisha Ltd
Current assignee: Usui Kokusai Sangyo Kaisha Ltd
Priority date: 1980-03-24
Filing date: 1981-03-20
Publication date: 1981-10-07
Also published as: BR8101746A; DE3111249A1; CA1178517A; JPS56133417A; AU6866881A; FR2482136A1

Abstract

The invention relates particularly to tubes or bars of iron or steel which after plastic deformation (typically in the range 20-60% reduction in area) resulting in a flattened grain structure, are annealed in such a way as to retain that grain structure. A hardness in the range 55-125% of the hardness of materials treated by full annealing can be achieved.

Description

SPECIFICATION Improvements in and relating to ferrous materials This invention relates to ferrous materials and in particular, tubes or bars of iron or steel which after plastic deformation have been subjected to low-temperature annealing in order to remove the residual stress generated during the plastic deforming step.

Conventional tubes or bars of iron and steel such as, those specified by the SAE Standard, "Fuel Injection Tubing - SAE J529b," which are annealed after plastic deformation by e.g. drawings assume a normal structure containing a re-crystallized inner structure and acquire a hardness approximating to the hardness of the raw material prior to plastic deformation. This is because it has been the practice to effect complete or full annealing. As a result, conventional materials of this kind have tended to suffer from degradation of fatigue strength as shown in Table 1 and by the curve (B) in the graph of Figure 1 comparing the fatigue properties. Also products fabricated from these materials tend to exhibit the early occurrence of cracks and fractures in fastened portions, joined ends, etc., owing to the impacts such as of bending and vibrations.

One object of this invention is to at least mitigate the above disadvantages.

According to this invention we propose a method of treating ferrous materials comprising subjecting the material to plastic deformation whereby a flattened grain structure is produced, and subsequently annealing the material in such a way as to retain the flattened grain structure. The invention also includes ferrous material which has been treated by the method according to the invention. Other features of the invention are set forth in the appendant claims.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings.

Figure 1 is a graph comparing the fatigue properties of products fabricated from iron or steel materials according to the present invention with those of products fabricated from conventional iron or steel materials.

Figure 2 is a photomicrograph showing the grain structure material according to the present invention (magnification - Xl 00).

Figure 3 is a photomicrograph showing the grain structure of conventional material (magnification - X100).

The iron and steel materials according to this invention, which preferably have a carbon content in the range of 0.05% to 0.45% (by weight) retain a flattened grain structure substantially free from recrystallization and acquire a hardness which is greater by 55% to 125% than the hardness acquired by iron and steel materials which are treated by full anealing and a notably enhanced fatigue strength as indicated by the curve (A) in the graph of Figure 1.This is achieved by fixing the reduction in area, (i.e. the reduction ratio) during low-temperature plastic deformation within the range 20% to 60% so that the material retains a uniform flattened grain structure and subsequent annealing by a low-temperature working at a temperature and for a period of time commensurate with the aforementioned ratio of treatment, namely, the highest temperature and the longest duration beyond which the recrystallization occurs.

The reason for specifying a hardness in the range mentioned above naturally concerns the outcome of the plastic deformation. When the increase of hardness fails to reach the lower limit of 55% over the level acquired by full annealing, the materials under treatment do not retain a uniformly worked, uniform flattened grain structure and, as the result, do not exhibit a notable enhancement of fatigue strength. When the increase in hardness exceeds 125% (i.e. is not increased in proportion to the ratio during plastic deformation) the grain boundary slippage in the flattened grain structure gains in intensity to such an extent that the grain boundary is degraded so eventually lowering the fatigue limit.

Example: A carbon steel pipe of machine structure grade (STKM-13A, killed steel) measuring 12.7 mm in outside diameter and 2.5 mm in wall thickness was used as a raw steel material. By subjecting this raw steel material to plastic deformation by drawing with the reduction ratio fixed at 48%, there was obtained a tube measuring 8.0 mm in outside diameter and 2.3 mm in wall thickness and Hv 202 in hardness. Subsequently, it was subjected to a low-temperature annealing treatment in a pusher type heating furnace under a non-oxidising atmosphere at 41 00C for 35 minutes. The final product exhibited a tensile strength of 67.6 kg/mm2, an elongation of 12% and a hardness of Hv 190.This product retained a flattened grain structure free from recrystallization as shown in the photomicrograph of Figure 2 and possessed fatigue properties shown in Table 1 and by the graph (A) in the graph of Figure 1.

The conventional product as indicated in Table 1 and by the curve (B) in the graph of Figure 1, was a tube exhibiting a tensile strength of 39.3 kg/mm2, an elongation of 42% and a hardness of Hv117 which was obtained by subjecting the tube measuring 8.0 mm in outside diameter, 2.3 mm in wall thickness and Hv202 in hardness and obtained by the same plastic deformation by drawing as mentioned above to a complete annealing treatment in a pusher type heating furnace under a non-oxidising atmosphere at 730"C for 10 minutes. This product possessed a recrystallized structure as shown by the photomicrograph of Figure 3.

Five test pieces taken from each of the products (A) and (B) were subjected to a repeated bending test for fatigue strength. The results were as shown below.

Method of measurement A test piece was held horizontally with one end fastened by a stationary holder and the remaining free end of the test piece was vibrated in a vertical direction.

wherein, a stands for the stated stress 4 stands for the distance the fastened point to the working point of force.

d stands for the diameter of the test piece, and E stands for the Young's modulus (2.1 x 1 04kg/mm2) of the material.

The test piece was given a repeated bending with the amplitude calculated by the formula (1) shown above, with count taken of the bends given to the test piece until a fracture was sustained by the test piece.

Test results TABLE 1 Stated Number of repeated bends given until fracture stress Product (A) of Convention product (B) (kg/mm2) this invention 30 Fracture sustained after Fracture sustained after 1.6 x 106 bends. 1.5 x 105 bends.

27 Fracture sustained after Fracture sustended after 3.8 x 106 bends. 2.4 x 105 bends.

24 No fracture sustained Fracture sustained after after 1.0 x 107 bends. 4.8 x 105 bends.

21 " " Fracture sustended after 1.1 x 105bends.

18 " " No fracture sustained after 1.0 X 107 bends.

As described above, owing to the annealing treatment which is performed in the form of a low-temperature working, the iron and steel materials produced according to the present invention retain a uniform flattened grain structure free from residual stress and incapable of recrystallization, and possess a hardness which is greater by 55% to 125% than that of conventional materials which have undergone complete annealing.

Consequently they enjoy a notable enhancement in the fatigue strength against bending and vibrations. The materials, when used in fabricated products such as tubes and bars, preclude occurrence of cracks and fractures in the fastened portions, joined ends and so forth. The products of this invention, therefore, prove highly useful for industrial applications.

Claims

1. A method of treating ferrous materials comprising subjecting the material to plastic deformation whereby a flattened grain structure is produced, and subsequently annealing the material in such a way as to retain the flattendd grain structure.

2. A method according to Claim 1, wherein annealing following plastic deformation is carried out at a particular temperature and for a period of time beyond which structural recrystallization occurs.

3. A method according to any one of the preceding claims, wherein the materials have a carbon content in the range 0.05% to 0.45% (by weight).

4. A method according to any one of Ciaims 1 to 3, wherein the plastic deformation is effected by drawing at a reduction ratio within the range 20% to 60%.

5. A method of treating ferrous materials substantially as hereinbefore described.

6. Ferrous material which has been treated by the method according to any one of the preceding claims.

7. Material according to Claim 6 wherein the flattened grain structure is substantially free from recrystallization.

8. Material according to Claim 6 or Claim 7, and in the form of a tube.

9. Material according to Claim 6 or Claim 7 and in the form of a bar.