DE2355894C3 - Process for warm forging of carbon steel or low-alloy steel - Google Patents

Description

25th

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 forging. While drop forging works at temperatures between 900 ° and Π 50 ° C ^., Cold forming works around room temperature. Both processes have their disadvantages and disadvantages. In closed-die forging, it is the smaller forces and fewer operations associated with lower accuracy and poor surface quality due to the formation of scale. In the case of cold forming, it is the greater accuracies combined with much greater deformation work and many more processing stages. When semi-hot forging is preferably performed in the range between 600 ° and 900 ⁰ C ( "Industrieanzeiger", 2259-2260 91. Born, No.93 from T.November 1969 side), whereby the disadvantages of the two above-mentioned methods reduce noticeably permit. The warm forging process, however, has the disadvantage that a heat treatment is required after forging in order to obtain the desired mechanical properties in the thus forged products; however, this heat treatment leads to deformations and residual stresses in the forged products and / or to deteriorated dimensional accuracy.

Ais of 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 in Range i> o from 450 ⁰ C to the A ₀ 1 conversion point (72 Γ C) is subjected. With this method, a high dimensional accuracy and good surface appearance is achieved similarly to the exclusive cold forming or pressing in that, apart from the 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 slight improvements in strength compared to an exclusively applied one Warm forging process achieved However, the many process stages and the high level are disadvantageous Tool load during the upstream cold forming, which requires a large amount of labor and high Means production costs or high tool wear

The object of the invention is therefore to provide a method for warm forging of carbon steel or To create low-alloy steel, in which the time-consuming, complex heat treatment after final cold hot forging is also avoided, but with fewer process steps and with less technical effort, such as B. tool wear, and in which the mechanical properties of the steel obtained, such as its Strength toughness Notched impact strength, creep strength, fatigue strength, the appearance and the Dimensional accuracy are also much improved.

According to the invention, this object is achieved by the features specified in the characterizing part of the main claim. An advantageous embodiment the invention can be found in the subclaim.

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 is achieved despite the lack of post-heat treatment. Weger / the one no longer required here After heat treatment, the dimensional accuracy and the surface of the forged products obtained are also improved. 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 for cold forging, which extends the service life of the Tools extremely shortened, avoided.

The warm forging in a relatively low temperature range of 450 ⁰ C up 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 Ac i transformation point and, if necessary, 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 forgings 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, optionally 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 A _c 1 and the Acs transformation point, then cooled again or subjected to hardening, the steel having both phases ferrite and austenite, and then

the warm forging carried out in the above-described temperature range of 450 ⁰ C to the Aci transformation point, the temperature of the starting steels is increased as a result of the forging heat generated during forging to a temperature close to the Ac3 transformation point or above. In warm forging, the austenite that has been formed before forging by the heat treatment of the present invention, the austenite that has been formed when the temperature rises due to the heat of forging in warm forging, a transformation, resulting in a structure that is relatively finer austenite grains Has. 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 treatment 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 kept above the Act 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, the temperature of the steels in the heat pretreatment according to the invention, which may include hot deformation, for example forging, should preferably be in the range from the A ₀ 3 transformation point -50 "C to the Ac 3 transformation point. which is followed by hot working, such as hot forging, is heated to a temperature above the Ac 3 transformation point, the temperature of these steels during hot working is raised too much to obtain a finer structure, resulting in no improvement achieved in terms of 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. Included 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 a part of the ferrite structure remains in these hardened steels 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 deformation can be carried out, that will

final warm forging of the steels at one

Temperature in the range of 450 ° C up to Ad-Um-

transition point carried out

If the forging temperature at this final

sequent semi-hot forging at 450 ⁰ C, that is in the blue range is broken, the processing resistance is increased; This temperature range is therefore the least suitable for the forming of steels. If, on the other hand, the forging temperature at final warm forging exceeds the Aci transformation point, the during the Warm forging temperature reached too high, whereby the effect achieved by the heat pretreatment according to the invention is neutralized, so that one does not have to do with the smiths produced The greater the heating rate and the shorter the heating time in the obtained, the improved toughness the above-mentioned warm forging temperature range, the better the mechanical properties are achieved during the final warm forging. In this regard, the heating rate should preferably be above 30 ° C / minute and the Heating time should be less than 40 minutes. In addition, the greater the strength, the greater the Cooling rate is, although conversely a lower cooling rate results in higher toughness. For this reason, in the case of the In extrusion forging, the extrusion speed is preferably above 5 mm / second. The cooling

However, the speed after the final warm forging should be as high as possible so that one the desired strength is obtained even if greater toughness can be achieved at a lower cooling rate. So are the aforementioned Factors are 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 ⁰ C to the Ac l transformation point is relatively low, so that significantly improved dimensional accuracy and improved surface appearance can be achieved can. The cooling rate used here obviously depends on the shape and the Dimensioning of the manufactured forged parts;

so therefore the cooling rate cannot be specified more precisely here. However, the cooling rate should be such that the steels after the final warming, carried out _{at a temperature above the A C 3 transformation point,}

forge have a martensite structure.

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

is an additional process and takes place on the already finished blank 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 type I (JlS

S 45 C). the treatment stages forging hardening - Warm forging and normal hardening - »have been subjected to tempering,

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

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 steel can generally be used that are 0.1 to 0.6% carbon, no more than 1.0% silicon, and no more than 2.0% manganese contain, or alloy steels that contain one or more types of alloying 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 Contains 0.5% niobium or the like.

Table 1 illustrates the chemical composition Settlement of the applied samples.

Table I.

Chemical composition of the samples (% by weight)

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

Table II 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 at a temperature in the region of 450 ° C up to the Ac i transformation point, warm forging been (invention); the other rod is only subject to heat treatment or plastic deformation (comparative example from the prior art).

Table II

Warm forging results

Steel type Treatment of the samples Stretch train strain Cross-section- Notch impact border strength 4M «) diminish- toughness (kp / mm ^:) (kp / mm ² ) (%) tion
(%) / mkp \
At ² I. Type I. 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 COO C starting - * 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 hardening 67.3 79.2 27.7 Ma 19.5 - 600 C warm forging 750 C forging hardness 67.5 80.0 23.2 60.5 22.5

- 600 "C warm forging

*) Comparative example from the prior art
**) 4 · \ jA = stretched 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 the semi-hot forging is carried out at 600 ⁰ C, the conditions of preparation treatments van wöbe be ated, forgings are obtained di <

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 with a diameter of 25 mm; the samples have been subjected to a rolling treatment, a heat treatment or a hardening with tempering, whereupon 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 Notched bar border strength 4 · [A **) diminish toughness tion / mkp \ IU n / mm-ϊ <° /. \ (%) 1 CfTl ² I. *) Hardening * tempering 64.3 Type I. Hardening - * ■ Warm forging 47.1 76.9 27.4 55.1 9.4 (JIS S45C) Hardening and tempering 93.7 107.4 17.0 58.5 12.7 - * Warm forging 89.5 101.0 18.2 13.3 rolled - * warm forging 35.6 *) Hardening - * tempering 95.0 105.4 13.0 71.8 6.0 Type II Hardening - * warm forging 63.0 73.6 25.6 61.2 18.5 (JIS SCM22) Hardening and tempering 101.9 103.9 19.8 64.8 20.9 - * Warm forging 95.2 98.5 20.6 22.5 rolled - »warm forging 51.2 82.8 95.8 15.9 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 C x 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 rolled steels at 600 "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 treatments while having 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 such

such as forgings that have been subjected to rolling and warm forging.

F i g. Figure 2 illustrates the results of an ONO-type rotary bending fatigue test on those steels which have been subjected to hardening and warm forging according to the invention in comparison with the results of such steels which have been subjected to the usual hardening and tempering treatments. As can be seen from FIG. 2, forgings which have been treated according to the invention show excellent values for fatigue strength which are 12 kgf / mm ² higher than the corresponding values for forgings which have been subjected to the usual hardening and tempering.

Table IV illustrates the mechanical properties of steel rods 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

ίο

Warm forging was carried out on the steels using bars 16 mm in diameter using a 1/2 ton 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

(kp / mm ² ) (kp / mm ² )

Elongation cross-section

4 · I / (diminish- tenacity

^run B / mkp \

(%) (%) learn ² '

Type I.
(JIS 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 (JIS S 45 C) with a diameter of 25 mm in a heating furnace is introduced, which at 820 ⁰ C (A ₃ + 27 ° C), 78o ⁰ 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 forgings obtained according to the invention have higher values for the Strength and toughness than those steels that

. '■ 5 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

jo grain structure back. F i g. 1 illustrated in photomicrographs the grain sizes (heat treatment grain sizes) of a No. 2 steel obtained according to the present invention and a steel with the number 5, obtained by the usual procedure

j) was. 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
border
(kp / mm ² ) train
strength
(kp / mm ² ) strain
4 ■ \ A
(%) cross-section
diminish
tion
(%) Notch impact
toughness
/ mkp \
At ² y 69.0 83.6 24.1 60.4 15.0 67.5 80.0 23.2 60.5 22.5 68.0 83.5 25.2 62.0 26.0 68.5 84.0 26.0 64.3 27.0

1 Type I 820 C forging hardening

(JIS S45C) - 600 ⁰ C warm forging

2 750 ⁰ C forging hardening

- · ■ 600 ° C warm forging

3 730 ⁰ C forging hardening

- 600 ° C warm forging

4 730 ⁰ C forging hardening
-► 600 "C tempering,

600 ^c C warm forging

5 *) 850 ^c C hardening

- 600 ^c C tempering

*) Comparative example from the prior art

61.0 76.9

28.0

61.5

11.6

The above exemplary embodiments follow the usual hardening treatments show that the hot forging according to the invention - tempering - normalizing or Schmiedehät least 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 Ac r transformation point (721 ⁰ C), characterized in that this steel is subjected to a pretreatment before the warm forging process Heating to a temperature above the Ac i transformation point, optionally hot working, and then subjecting to cooling. 2. Process according to Claim 1, characterized in that the steel is subjected to heating to a temperature in the range between the A ₀ I transformation point and the Acs transformation point, preferably between the A _c r transformation point minus 50 ⁰ C and the Acs transformation point will.