DE2355894B2 - Process for warm forging of carbon steel or low-alloy steel - Google Patents

Description

The invention relates to a method for warm forging of carbon steel or low-alloy steel Steel according to the preamble of the main claim.

Warm forging lies in the range between conventional closed-die forging and cold forming. While drop forging works at temperatures between 900 ° and 1150 ° C, it is used worked at about room temperature for cold forming. Both methods have their advantages and disadvantages. In closed-die forging, it is the smaller forces and fewer operations associated with lower ones Accuracy and poor surface quality due to scale formation. When it comes to cold forming, they are greater accuracy combined with much greater deformation work and many more processing stages. Warm forging is preferably carried out in the range between 600 ° and 900 ° C ("Industrieanzeiger", Volume 91, No. 93 of November 7, 1969, pages 2259 to 2260), whereby the Disadvantages from the two processes mentioned above can be reduced noticeably. The warm forging process however, it has the disadvantage that a post-forging heat treatment is required to achieve the desired one preserve mechanical properties in forged products; this heat treatment but leads to deformations and residual stresses in the forged products and / or deteriorated Dimensional accuracy.

From the journal "Industrieanzeiger", 92nd Volume, No. 65 of August 7, 1970, pages 1566 to 1567, a method for warm forging of carbon steel or low-alloy steel is known, the steel being pretreated by cold forming and then warm forging at a temperature im Range from 450 ° C to the A _c i transition point (721 ° C) is subjected. With this method, a high dimensional accuracy and good surface appearance is achieved similar to that with exclusive cold forming or pressing in that, apart from stress-relieving annealing at approx. 650 ° C., no heat treatment is required after the warm forging process. In addition, this cold forming before warm forging achieves slight improvements in strength compared to an exclusively used warm forging process. However, the many process stages and the high tool load during the upstream cold forging, which means a large amount of time and high production costs or high tool wear, are disadvantageous
The object of the invention is therefore to provide a method

ίο for warm forging of carbon steel or To create low-alloy steel, in which the time-consuming, costly heat treatment after the final cold hot forging is also avoided is, but with fewer process steps and with less technical effort, such as. B. Tool wear, comes from and in which the mechanical properties of the steel obtained, such as its Strength, toughness, notched impact strength, creep strength, Fatigue strength, appearance and dimensional accuracy are also significantly improved. According to the invention, this object is given in the characterizing part of the main claim Features solved An advantageous embodiment of the invention can be found in the dependent claim.

The forged products produced by the process according to the invention have the required properties mechanical properties, with an excellent combination of improved mechanical Properties despite the lack of post-heat treatment

jo is reached. Because of the no longer required here Post heat treatment is also the dimensional accuracy and the surface of the forged products obtained Furthermore, the manufacturing process is extremely shortened and simplified because a series of pre-treatment stages such as tempering and soft annealing before cold forging at The known methods explained above are omitted and post-heat treatment is also unnecessary the high tool stress that is typical of cold forging, which extends the service life of the Tools extremely shortened, avoided.

The warm forging in a relatively low temperature range from 450 ⁰ C to the Ac i transformation point can also suppress the strong shrinkage of the workpiece that occurs at high forging temperatures, while the heat pretreatment carried out in the method according to the invention before the actual warm forging in the form of heating to a Temperature above the A _c ι transformation point and possibly hot deformation, the post-heat treatment of the manufactured forgings can be avoided, so that the forgings not only have excellent dimensional accuracy but also an excellent appearance! With minimal scale formation on their surface. The finished forged parts are free from warping or warping and from residual stresses.

The heat pretreatment according to the invention before

The final warm forging can consist of heating, possibly in connection with hot deformation, of hardening or of hardening and tempering treatments. The rolled or normalized starting steels are heated to a temperature between the Ac ι- and the A _C 3 transformation point, then cooled again or subjected to hardening, the steel having both phases ferrite and austenite, and then

The warm forging is carried out in the temperature range described above from 450 ⁰ C to the Aci transformation point, the temperature of the starting steels being increased as a result of the forging heat generated during forging up to a temperature close to the Acs transformation point or above. In warm forging, the austenite that was formed prior to forging by the heat treatment according to the invention, inter alia, the austenite that was formed when the temperature increased as a result of forging, caused a transformation, resulting in a structure that is relatively finer Austenite grains. When steels forged in this way are cooled, a finer martensite structure is obtained. If desired, additional hardening and tempering treatments can also be carried out after the heat pretreatment mentioned and before the final warm forging. According to the invention, the tempering treatment can also be carried out simultaneously during the hot-working process, so as to dispense with a separate tempering treatment, which results in considerable advantages in the production of forgings.

If, according to the method according to the invention, the steels in their heat pretreatment before the final warm forging must also be hot-formed, the heating temperature should be high enough to austenitize the steels as a result of the heat of forging. For this reason, the temperature of the steels to be heated should be above the Ac; The transformation point of the steels. if however, the rate of hot deformation, i.e. H. the forging speed, or machining speed, in the preparatory heat treatment stage is relatively low, you get a lower one, through the The heat of the forging caused the steels to be austenitized, resulting in the desired toughness and No longer strength even by performing heat treatment after warm forging can be reached.

For this reason should the temperature of the steels in the inventive heat treatment, which may include a hot working, such as forging, preferably in the range from the A _c3 transformation point -um- -50 ⁰ C to the Ac 3 Utnwandlungspunkt lie. On the other hand, if this pre-heat treatment, followed by hot working such as hot forging, is heated to a temperature above the Acs transformation point, the temperature of these steels during hot working is raised too much to obtain a finer structure; this results in no improvement in the toughness of the steels

The steels which, as part of the heat pretreatment according to the invention, are heated, if necessary have been subjected to hot working such as hot forging then subjected to cooling hardening so as to form a fine martensite structure. Thereby wi oil cooling can be used for hardening after this preparatory hot forging. If the However, if steels are small in shape, air cooling can also be used.

Even if the steels are not completely austenitized in the preparatory hot forming stage have been, it is harmless if part of the ferrite structure remains in the steels thus hardened are eventually tempered and normalized to give the desired strength and toughness

Subsequent to this heat pretreatment according to the invention, optionally in conjunction with a Hot working can be carried out, the final warm forging of the steels is carried out at a Temperature carried out in the range of 450 ° C to the Ad transformation point.

If the forging temperature in this final warm forging is below 450 ^° C., ie in the blue fracture range, the machining resistance is increased; This temperature range is therefore the least suitable for the forming of steels.If, on the other hand, the forging _{temperature during the final warm forging exceeds the Ac 1} transformation point, the temperature reached during warm forging becomes too high, which neutralizes the effect achieved by the heat pretreatment according to the invention, so that the improved toughness is not obtained in the produced forged parts.The greater the heating rate and the shorter the heating time in the warm forging temperature range mentioned above, the better the mechanical properties are achieved in the final warm forging be less than 40 minutes. In addition, the greater the cooling rate, the greater the strength, although conversely, a lower cooling rate results in higher toughness. For this reason, in the case of extrusion forging, the extrusion speed is preferably above 5 mm / second. The cooling rate after the final warm forging should, however, be as high as possible so that the desired strength is obtained, although greater toughness can be achieved at a lower cooling rate. The above-mentioned factors should be determined depending on the requirements of the respective forged parts to be manufactured.

The temperature used in the final warm forging in the range from 450 ^° C. to the Ad transformation point is relatively low, so that significantly improved dimensional accuracy and improved surface appearance can be achieved. The cooling rate used here obviously depends on the shape and dimensions of the manufactured forged parts; therefore the cooling rate cannot be specified here in more detail. However, the cooling rate should be such that the steels have a martensite structure after the final warm forging carried out _{at a temperature above the A C 3 transformation point.}

It should be expressly pointed out that the heat and / or hot deformation treatment according to the invention is a contrast to the usual treatment for producing a blank for warm forging additional process is and takes place on the already finished blank, i.e. with the production of the blank has nothing to do.

The invention will now be explained in more detail using exemplary embodiments in conjunction with the drawing will; it shows

F i g. 1 Microphotographs of the structures with hardened and tempered and also heat-treated grain sizes in steels of Tvd I (HS

S 45 C), which have been subjected to the treatment stages forging hardening - <■ warm forging and normal hardening - * tempering,

Fig. 2 is a graphic representation for illustration the results of an ONO-type rotary bending fatigue test on rolled steels of type I (JIS S 45 C), the treatment stages hardening -► warm forging and the usual hardening - ► have been subjected to tempering.

Steels that can be used in the method according to the invention include steels with low or medium carbon content and

low-alloy steels, as well as any other steels that can be used for forging. However Carbon steels can generally be used that contain 0.1 to 0.6% carbon, not more than 1.0% silicon and not more than 2.0% manganese, or alloy steels that contain one or more Types of alloy elements at not more than 4% nickel, not more than 2% chromium, not more than 1.0% Molybdenum, not more than 0.05% boron and not more than 0.5% niobium or the like. Contain.

Table I illustrates the chemical composition of the samples used.

Table I.

Chemical composition of the samples (% by weight)

Steel type C. Si Mn P. S. Cu Ni Cr Mon Type I.
(JIS S45C)
Type II
(JIS SCM22) 0.47
0.20 0.22
0.30 0.79
0.75 0.011
0.016 0.012
0.008 0.01
0.04 0.02
0.04 0.11
1.06 0.20

Table Π illustrates a comparison of the mechanical properties of two different treated Type I steel bars (JIS S 45 C) with a diameter of 25 mm; one rod is after subjected to the method according to the invention of a heat treatment or plastic deformation

and then warm-forged at a temperature in the range of 450 ° C to the A _c l transformation point (invention); the other rod has only been subjected to heat treatment or plastic deformation (comparative example from the prior art).

Table Il

Warm forging results

Steel type Treatment of the samples Stretch train strain cross-section Notch impact border strength 4 fÄ **) diminish toughness tion / mkp \ (kp / mm ² ) (kp / mm ² ) (%) (%) At ² y Typi Hardening (JIS S45C) *) 850 C hardening, 61.0 76.9 28.0 61.5 11.6 600 C tempering 850 C hardening 58.5 75.4 26.9 65.3 15.8 -► 600 C warm forging 850 C hardening, 67.5 80.0 28.1 67.9 20.3 600 C tempering - 600 C warm forging Forging hardening *) 850 C forging hardness 67.4 77.9 23.2 62.6 16.5 - * 600 "C tempering 850 "C forging hardness 67.3 79.2 27.7 64.2 19.5 -► 600 "C warm forging 750 "C forging hardness 67.5 80.0 23.2 60.5 22.5

• 600 " ⁵ C warm forging

*) Comparative example from the prior art **) 4 - ψλ = stretching length of the test piece, where A denotes the cross section of a parallel area of the test piece

The notched impact strength values are significantly improved by using warm forging at 600 ^° C. It can be seen that when warm forging is carried out at 600 ^° C., the conditions of the preparatory treatments being varied. Forgings are obtained that

have excellent mechanical properties without the need for heat treatment after final warm forging would be used.

The preparatory treatments are also described in detail.

Table III illustrates the mechanical properties of samples as shown in Table I having a diameter of 25 mm; the samples have been subjected to a rolling treatment, a heat treatment or a hardening with tempering, after which these samples are subjected to the final hot extrusion forging up to a diameter of 16 mm at a temperature of 450 ° C up to the A _c i transformation point at a working speed of 20 mm / sec, while water cooling was then carried out (this procedure relates to a particular embodiment of the invention).

Table III

Mechanical properties of warm extrusion forged steels

Steel type Treatment of the samples Stretch train strain cross-section Kerbschlae- border strength 4 ■ \ fÄ **) diminish toughness tion / mkp \ (kp / mm ² ) (kp / mm ² ) (%) (%) At ² y

Type I *) Hardening - tempering 47.1 76.9 27.4 64.3 9.4

(JIS S45C) Hardening - Warm Forging 93.7 107.4 17.0 55.1 12.7

Hardening and tempering 89.5 101.0 18.2 58.5 13.3

-► Warm forging

rolled - warm forging 95.0 105.4 13.0 35.6 6.0

Type II *) Hardening - tempering 63.0 73.6 25.6 71.8 18.5

(JIS SCM22) Hardening - Warm Forging 101.9 103.9 19.8 61.2 20.9

Hardening and tempering 95.2 98.5 20.6 64.8 22.5 -► Warm forging

rolled - warm forging 82.8 95.8 15.9 51.2 13.8

*) Comparative example from the prior art
**) see footnote to table II

The following conditions were used for hardening ^: and tempering:

Hardening

Tempering

Type I (JIS S45C)
Type II (JIS SCM22)

850 C x 30 min.
880 CX 30 min.

600 CX 30 min.
600 C x 30 min.

The abbreviations used in the tables have the following meanings:

Hardening »warm forging:
After curing, heating may be employed at 600 ⁰ C for 30 minutes and hot extrusion forging;

Hardening - tempering - ► warm forging:
After curing, the tempering, heating is carried out at 600 ⁰ C for 30 minutes and hot extrusion forging;

rolled- »warm forging:

Heating of rolled steels at 600 ⁰ C for 30 minutes and hot extrusion forging.

Show the forgings that have been subjected to hardening (tempering) and warm forging slightly lower values for elongation and percentage reduction in area than those for one Have been subjected to hardening and tempering treatment, while they have higher values for tensile strength, Exhibit yield strength and impact strength than the latter. Forgings that are subjected to hardening and afterwards have been subjected to warm forging according to the invention, show significantly improved Ductility and toughness compared with forgings subjected to rolling and warm forging have been subjected.

Fig. 2 illustrates the results of an ONO-type rotary bending fatigue test in those steels which have been subjected to hardening and warm forging according to the invention, in comparison to the results of those steels which have been subjected to the usual hardening and tempering treatments are. As can be seen from Fig.2, show Forgings that have been treated according to the invention,

to excellent values for the fatigue strength, which was 12 kp / mm ² higher than the corresponding values for those forgings that have been subjected to the usual hardening and tempering

Table IV illustrates the mechanical properties of steel bars of type I (JIS S 45 C) with a diameter of 25 mm each, the one Have been subjected to forging under the same conditions as previously described, the

10

Warm forging was carried out on the steels using bars 16 mm in diameter using a '^ barrel hammer at 60 cycles / min.
Open-die forging technology to achieve rounds

Table IV

Mechanical properties of semi-hot forged steels compared to conventionally hardened and tempered steels

Steel type

Treatment of the samples

Yield strength tensile strength

strain

(kp / mm ² ) (kp / mm ² ) (%)

Cross-sectional impact reduction toughness

tion

/ mkp \
At ² y

Type I.
(JlS S45C)

*) Hardening - tempering 47.1 76.9 27.4 64.8 9.4

Hardening - warm forging 70.0 85.4 25.8 63.5 13.3

*) Comparative example from the prior art

As can be seen from Table IV, steels that have been subjected to open die forging also offer improved values for toughness and strength as in the case of extrusion forging.

According to a further particular embodiment of the invention, samples of steels of type I (J'S S 45 C) with a diameter of 25 mm each were introduced into a heating furnace which was operated at 820 ^° C. (A ₃ + 27 ^° C.), 780 ^° C. ( A ₃ -13 ° C) or 750 ° C (A ₃ -43 ° C) was maintained; This was followed by heating for 30 minutes and open die forging up to a diameter of 20 mm and immediately afterwards forging and cooling in water. The steels treated in this way were then ^{heated to 600 °} C. for 30 minutes and subjected to warm forging up to a diameter of 16 mm. The mechanical properties obtained are shown in Table V below.

As can be seen from Table V, the Schmiodeteile obtained according to the invention have higher values for the Strength and toughness than those steels which are subjected to the usual hardening and tempering treatments have been; in particular, the notched impact strength values are twice those of the latter materials. The properties of the steels obtained according to the invention are finer Grain structure back. F i g. 1 illustrates the grain sizes (heat treatment grain sizes) in photomicrographs from a steel No. 2 obtained according to the invention and from a steel No. 5 obtained by the usual method. This compilation shows that a fine grain size (heat treatment grain size) in the steels (No. 2) obtained according to the present invention were generated.

Table V

Mechanical properties

No. steel type

Treatment of the samples

Stretch limit

tensile strenght

(kp / mm ² ) (kp / mm ² )

Elongation cross-section- notched impact-4 · | /, · ί decrease- toughness

™ ⁿ Z / mkp \

(%) (%) By ² y

1 Type I 820C forging hardening

(JIS S45C) - 600 ⁰ C warm forging

2 750 ° C forging hardness

- * ► 600 ° C half-tank forging

3 730 ° C forging hardening

- ♦ 600 ⁰ C warm forging

4 730 ° C forging hardening

- 600 ^D C tempering,

600 ° C warm forging

5 *) 850 ⁰ C hardening

- 600 ⁰ C tempering

*) Comparative example from the prior art

69.0 83.6 24.1 67.5 80.0 23.2 68.0 83.5 25.2 68.5 84.0 26.0

76.9

28.0

60.4
60.5
62.0
64.3

61.5

15.0
22.5
26.0
27.0

11.6

The above exemplary embodiments follow the usual hardening treatments shows that the Wannschmiedever- - tempering - normalizing or forging hardening driving leads to excellent forged steels, could not get.

For this purpose 2 sheets of drawings

Claims (2)

Patent claims: 1. Process for warm forging of carbon steel or low-alloy steel, which is pretreated and then ^{subjected to warm forging at a temperature in the range from 450 0} C to the Aci transformation point (721 ° C), characterized in that this steel is pretreated before the warm forging process heating to a temperature above the Ac ι transformation point, optionally hot forming, and then cooling is subjected. 2. The method according to claim 1, characterized in that the steel is subjected to heating to a temperature in the range between the Aci transformation point and the Acs transformation point, preferably between the Ac3 transformation point minus 50 ° C and the A ₀ r transformation point.