US3844852A - Method of heat treatment of housings - Google Patents

Abstract

A method of heat treating housings wherein the low-stressed housing zones located in the side wall of the housing near spring pads are heated to 800* - 1250* C in the course of the time during which the housing zones are subjeted to maximum tensile stresses, and that which is located near spring pads in the lower wall of the housing are heated due to heat conduction to a temperature of not over 300* C after which said housing is cooled in the air in the course of at least 180 s.

Description

United States Patent 91 Krendel et al.

[ Get. 29, 1974 METHOD OF HEAT TREATMENT OF HOUSINGS [76] Inventors: Leonid Avrelievich Krendel, ulitsa Gagarina 24, kv. 14, Serdobsk Penzenskoi oblasti; Vladimir Filippovich Nikonov, ulitsa Avtozavodskaya 1 1/1, kv. 125, Moscow; Viktor Alexeevich Ognevsky, ulitsa Krasnogo Mayaka, 8, korpus 1, kv. 22, Moscow; Anatoly Georgievich Orlovsky, ulitsa Simferopolskaya, 14/2, kv. 21, Moscow; Grigory Arkadievich Ostrovsky, Dimitrovskoe shosse, 41, kv. 125, Moscow; Valery Evseevich Palkin, Samarkandsky bulvar 6, korpus 2, kv. 190, Moscow; Anatoly Vladimirovich Pakhomov, ulitsa Verkhnyaya Maslovka 28, kv. 10, Moscow; Anatoly Loginovich Stepin, ulitsa Birjulevskuya, 1, korpus 2, kv. 347, Moscow; Oleg Fedorovich Troilmov, ulitsa Chusovskaya 1 l, korpus 8, kv. 92, Moscow; Konstantin Zakharovich Shepelyakovsky, ulitsa Avtozavodskaya, 6, kv. 125, Moscow; Alexandr Moiseevich Ryskind, ulitsa Kirovogradskaya, 24, korpus l, kv. 161, Moscow; Isaak Nakhimovich Shklyarov, ulitsa Velozavodskaya, 9, kv. 24, Moscow; Moisei Osipovich Rabin, Simonovsky val, korpus 1, kv. 119, Moscow; Veniamin Davydovic'h Kalner, Lomonosovsky prospekt,

15, kv. 137, Moscow; Mikhail Mironovich Fishkis, Zvezdny bulvar, 5, kv. 28, Moscow, all of USSR.

[22] Filed: Dec. 27, 1972 [21] Appl. No.: 319,044

Primary ExaminerC. Lovell Attorney. Agent, or Firm-Holmun & Stern [57] ABSTRACT A method of heat treating housings wherein the lowstressed housing zones located in the side wall of the housing near spring pads are heated to 800 1250C in the course of the time during which the housing zones are subjeted to maximum tensile stresses, and that which is located near spring pads in the lower wall of the housing are heated due to heat conduction to a temperature of not over 300C after which said housing is cooled in the air in the course of at least 180 s.

3 Claims, 7 Drawing Figures METHOD OF HEAT TREATMENT OF HOUSINGS The present invention relates to methods of the heat treatment of automobile driving axles and can be utilized to advantage in automotive industry.

It is known that automobile driving axle housings are subjected to the effect of asymmetric cyclic loads.

Under these circumstances the lower wall of the housing has zones located close to the spring pads which are subjected to the effect of maximum tensile stresses. At the same time the working stresses applied to the side walls near the neutral plane of the housing are very low. The zones of maximum stresses are strengthened by a known method of heat treatment in which the entire housing is hardened and then tempered.

In this known method the entire housing is heated to above the critical temperatures and then cooled in oil or water.

However, this through hardening of the housing leaves residual stresses in its superficial layers, i.e. at the points to be subjected to maximum tensile stresses. This impairs the fatigue strength of the housing. Another disadvantage of the known method for heat treatment lies in the high cost of the process due to a large consumption of electric power for heating the housing height.

and the high expenditures for refractory materials and quenching oil.

Besides, the known method of heat treatment calls for extensive production areas.

An object of the present invention resides in the elimination of the aforesaid disadvantages.

The main object of the present invention is to provide a method of heat treating automobile driving axle housings which would produce residual compressive stresses at the points to be subjected to maximum cyclic tensile stresses, thereby extending the overall service life of the housings.

This object is accomplished by providing a heat treatment method for automobile driving axle housings which are to be subjected to the effect of asymmetric cyclic loads and which have zones located close to the spring pads on the lower wall of the housings carrying the maximum tensile stresses, and other zones located in the middle of the side walls and carrying low stresses wherein, according to the invention, the low-stressed zones are heated to from 800 to 1250C for a certain time period during which the zones being subjected to maximum tensile stresses are heated by conduction to not over 300C after which the entire housing is cooled in the air for at least 180 s.

This method of heat treatment improves the strength and durability of the housing and cuts down the consumption of refractories, quenching oil and electric power.

According to the invention, the low-stressed housing zones are heated for 180 s or less.

This time is sufficient to prevent the zones being subjected to maximum stresses from being heated in excess of 300C due to heat conduction.

Before heating the low-stressed zones of the housing it is possible, according to the invention, to harden the zones to be subjected to maximum tensile stresses.

The hardening of these zones will strengthen the lower wall still further.

These dimensions of the afore-mentioned zones of the housing will ensure a maximum increase in the housing life.

The embodiments of the invention are set forth with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic view of the housing subjected to heat treatment according to the invention;

FIG. 2 is a diagram of bending moments acting on the housing in service;

FIG. 3 is a section taken along line IIIlII in FIG. 1;

FIG. 4 is a section taken along line IV-IV in FIG. 1;

FIG. 5 is a diagram showing the distribution in height of stresses applied to the housing in service;

FIG. 6 is a housing heating chart used in the heat treatment method according to the invention;

FIG. 7 is a diagram showing the distribution of residual stresses on the lower wall of the housing after heat treatment according to the invention.

The housing 1 (FIG. 1) of the automobile driving axle rests on wheels at points A and A and points B and B nearby carries the weight of the automobile through spring pads.

A diagram of the bending moments acting in various sections of housing I is shown in curve m (FIG. 2). The bending moment (curve m) grows from points A and A to points B and B respectively, whereas the points between B and B it has a constant maximum value. However, the section of the housing 1 between these points varies greatly. The section and resistance of the housing grow sharply in its middle part where the final drive is installed. As a result, the stresses acting in the middle of the housing are rather low and the danger of failure is negligible.

The lower wall 2 (FIGS. 3, 4) of the housing I is most susceptible to fatigue failure in the zones located close to the spring pads, particularly so near the transverse welds 3 where there is a high concentration of stresses.

The same sections in the side walls 4 (FIGS. 3, 4) of the housing 1 near the longitudinal axis 5 (FIG. 1) have zones 6 subjected also being to low stresses in service.

Indeed, curve n (FIG. 5) indicating the distribution of stresses along the height of the side wall 4 (FIGS. 3, 4) of the housing shows that the stresses pass through the zero value near the longitudinal axis of the side wall.

According to the invention, the low-stressed zones 6 (FIG. 1) are heated to from 800 to l250C. The best solution is when the zones 6 on both side walls 4 are located above zones FG which are subjected to the effect of maximum tensile stresses, and at some distance from the housings lower wall, said distance being equal to 0.4-0.7 of the housing height.

The height of the zones 6 is 0.2-0.5 of that of the housing height whereas their length depends on the length of the housing zones susceptible to failure and may vary, correspondingly, to from I to 4 of the housing heights near the spring pads. Such localized heating can be effected with the aid of any conventional means, such as induction currents, contact electrical heating or gas torch means.

Assuming that the housing 1 is I20 mm high and the zone of probable failure located on its lower wall 2 is 250 mm long, thus the zones 6 would be located at a height of 80 mm and will be 50 mm wide and 300 mm long.

According to the invention, the zones 6 (FIG. I) are heated to from 800 to l250C within the time period for which the zones FG being subjected to maximum tensile stresses cannot be heated by conduction above 300C.

FIG. 6 shows the heating curves of the zones 6 and FG (FIG. 1) of the housing 1, with the horizontal axis indicating the time in seconds while the vertical one gives temperature in degrees Centigrade.

As we can see from FIG. 6, the low-stressed zones 6 (FIG. 1) reaches a peak when heated for 65 seconds to I IC (curve f in FIG. 6) while the zones FG (FIG. 1) when subjected to maximum tensile stresses are heated by conduction only to 150C (curve g in FIG. 6).

The metal of the zones 6 being heated to l I0OC expands but the adjacent less-heated zones (the temperature difference exceeding 900C) oppose this expansion thereby causing plastic deformation of the heated metal.

After slow cooling, the metal in the zones 6 which were heated to 1 100C will be relatively smaller in size than that in the zones which remained comparatively cool.

As a result, the metal of the heated zones will be stretched while that in the adjacent zones being subjected to maximum tensile stresses will be compressed.

It thus follows from the above that a large difference in the heating temperatures of the zones being subjected to low and maximum tensile stresses is an indispensable prerequisite for the formation of stresses.

When the heating of the zones 6 is discontinued, the entire housing is cooled in the air over a period of at least 180 s.

During this period the temperature of the zones 6 (FIG. 1) drops (curve f in FIG. 6) while that of the zones FG (FIG. I) continues to rise (curve gin FIG. 6) due to the conduction of heat from the zones 6.

Thus, slow cooling in the air, as shown in FIG. 6, equalizes the temperature of all the housing zones at a temperature of about 400C.

The low heating temperature of the entire housing (400C) makes it possible to retain a comparatively high level of residual stresses.

If, however. the zones are heated to llO0C and quickly cooled in water or oil. they may be hardened which will increase their volume and stretch the zones being subjected to maximum tensile stresses this being unallowable.

The distribution of the residual stresses along the housing section after the heating described above and after cooling in the air, curve K as measured by electrical resistance strain gauges, is shown in FIG. 7. The residual stresses marked by the numerals in FIG. 7 are expressed in kg/mm with the compressive stresses being shown by the plus sign (-I-). It appears from FIG. 7 that the heat treatment according to the invention produces the desired distribution of residual stresses on the lower wall 2 which is to be subjected to maximum tensile stresses and will cause residual compressive stresses reaching 15 kg/mm".

The residual tensile stresses arising in the heated zones 6 (FIG. 1) are not dangerous since said zone 6 is located in the region of low tensile or compressive stresses; at the same time the residual compressive stresses arising on the lower wall 2 of the housing weaken the tensile stresses applied to the housing during operation, thus prolonging the housing life.

The strength characteristics of the housing can be further improved if the zones FG (FIG. 1) to be subjected to maximum tensile stresses are hardened before heating of the zones 6.

In this case the lower wall 2 (FIGS. 1, 3, 4) of the housing in the zone PG and the adjoining zones of its side walls 4 (FIGS. 1, 3, 4) become hardened throughout, with of said zones being from 0.2 to 0.3 of that of the housing height. The zones of the lower wall and the side wall zones adjacent thereto may be hardened by known methods of heating above the side critical points, usually above 850C for steel materials employed in the production of housings, followed by immediate cooling in oil or water.

The localized heating for hardening may be effected by any known means, such as by induction currents.

This kind of hardening produces residual compressive stresses in the lower wall 2 of the housing in the zones of maximal service stresses. The residual compressive stresses are formed in the lower wall during localized hardening thereof, due to a great volume of martensite developed as a result of hardening relative to the surrounding initial structure. Subsequent heating of the low-stressed zones 6 (FIG. 1) imposes additional residual 'compressive stresses on the zones FG.

There is no need for special tempering of the hardened zones of the housing lower wall 2.

The heating of the zones 6 to high temperatures followed by air cooling results in the heating of the entire housing 1 to I00C which is sufficient for tempering the hardened zones.

This method of heat treatment produces favourable distribution of strength characteristics and residual stresses throughout the housing section; thus, the lower wall 2 (FIGS. 1, 3, 4) of the housing (in the zones of probable failure) has the structure of tempered martensite characterized by a high strength and high yield limits and possesses sufficient residual compressive stresses which raise the resistance of the housing to asymmetric cyclic loads.

In some cases the low-stressed zones 6 (FIG. 1) can be heated after hardening the entire housing.

In this case, just as in the local hardening, there is no need for tempering the housing in the zones of maxi mum tensile stresses for the same reasons as above.

The service life of the housings after the heat treatment according to the present invention is more than doubled as compared with the life of the housings subjected to heat treatment according to the methods known heretofore.

Tests have shown that the housings heat-treated according to the known procedure which are subjected to cyclic loads varying from 0 to 17 tons have endured from 700,000 to l,000,000 cycles whereas the housings heat-treated according to the present invention have height from the housing lower wall, said zones having a length of from 1 to 4 times the housing height near said spring pads and a height of from 0.2 to 0.5 of said housing height near the spring pads, said zones of the housing side walls being heated at said temperature for not more than 180 seconds wherein the zones of said lower wall of the housing carrying maximum tensile stresses are heated by conduction to a temperature of not above 300C, and cooling the housing in air'for at least l80 sec.

2. The method of claim 1 wherein said heating of said zones of the housing is carried out after hardening of the zones of the lower walls and side walls adjacent thereto located near the spring pads and having a length of from i to 4 times the housing height near said spring pads and a height of from 0.2 to 0.3 of the housing height near said spring pads for said zones of the side walls.

3. The method of claim 1 wherein the heating of said zones of the housing is carried out after hardening of the entire housing.

Claims (3)

1. A METHOD OF HEAT TREATMENT OF AUTOMOBILE DRIVING AXIE HOUSINGS SUBJECTED TO ASYMMETRIC CYCLIC LOADS WHICH COMPRISES HEATING THE HOUSING SIDE WALL ZONES ADJACENT TO SPRING PADS TO A TEMPERATURE OF 800* TO 1250*C AT A HEIGHT OF FROM 0.4 TO 0.7 OF THE HOUSING HEIGHT FROM THE HOUSING LOWER WALL, SAID ZONES HAVING A LENGTH OF FROM 1 TO 4 TIMES THE HOUSING HEIGHT NEAR SAID SPRING PADS AND A HEIGHT OF FROM 0.2 TO 0.5 OF SAID HOUSING HEIGHT NEAR THE SPRING PADS, SAND ZONES OF THE

2. The method of claim 1 wherein said heating of said zones of the housing is carried out after hardening of the zones of the lower walls and side walls adjacent thereto located near the spring pads and having a length of from 1 to 4 times the housing height near said spring pads and a height of from 0.2 to 0.3 of the housing height near said spring pads for said zones of the side walls.

3. The method of claim 1 wherein the heating of said zones of the housing is carried out after hardening of the entire housing.

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