Process for high-temperature carburizing treatment of track bushes for tractors or tracked vehicles, with carburizing atmosphere produced in situ, direct quenching, annealing at ends and total stress relieving

Description

[0001] This invention refers to the production of bushes for the tracks of tracked vehicles in general, such as tractors and earthmoving machinery, more precisely, the invention refers to a production process in which the bushes are carburized at high temperature in a carburizing atmosphere produced in situ (using techniques which are already known as such but which have never been applied simultaneously to the items in question), and then are individually quenched and finally undergo a tempering of the ends and a total stress relief.

[0002] The bush, rotatably coupled to a pin, forms the articulated junction element between the track links.

[0003] During the relative movement between two connected track elements the pin rotates inside the bush with sliding friction and is subject to high dynamic loads applied directly to the bush by the driving sprocket. The variable temperatures under which earth-moving machinery operates further increase the stress on the bushes.

[0004] The technological properties the bush must have in order to withstand such use conditions include high surface hardness combined with good tensile strength, good fatique strength and good impact strength.

[0005] These properties are ensured by specifications that call for a completely martensitic structure in the centre and on the surface, 5-8 ASTM austenitic grain, surface hardness in excess of 55 HRC, centre hardness of 32-44 HRC and surface hardness at the ends of 35-46 HRC.

[0006] These requisites are obtainable by using a low-carbon steel (0.13-0.24% C) containing boron (0.0005-0.003% B) with 0.60-0.90f, Mn and 0.10-0.30% Si, both for the extrusion and for the machining processes of manufacture. Other elements such as aluminium, for instance, are preferentially added in the quantities normally required for the purpose of inhibiting the growth of austenitic grain during high-temperature carburizing.

[0007] However, the above steel composition is not a limiting factor as regards this invention; indeed, any steel capable of meeting the technical requirements indicated earlier is suitable for treatment by the present invention.

[0008] Attainment of the desired mechanical properties is an essential condition for the production of parts, such as bushes, subject to wear and decidedly unusual stresses, when in use. For these reasons, altough a number of treatments of great interest from the technical and economic aspects were already known, such as high-temperature carburizing and direct quenching, it had hitherto been considered impossible or anyway too complex and costly to apply them to the production of track bushes for tracked vehicles (henceforth simply referred to as bushes). In fact, with the higher temperatures involved in high-temperature carburizing, not only is there the risk of uncontrolled grain growth (a risk that in the case in point is controlled by the special inhibitors mentioned above), but there is also the risk that they may create temperature profiles and hence stresses in the pieces during cooling, such as to result in distortion and rupture or at least to lead to lack of uniformity of properties. Then again, direct quenching, which uses the residual carburiz ing heat, performed hitherto either on very small parts, treated en bloc or even sometimes on large parts, treated singly right from the carburiz ing heating phase, seemed difficult to apply to parts such as bushes, which are too small to be suitable for individual treatment right from the carburizing process, but too large to support direct quenching treat ment en masse, since any differences in packing them into the baskets can easily cause differences in cooling rates and hence in the mechanical properties, among distant parts of the bush or between the outer and inner surfaces thereof.

[0009] Up to now, the bush-manufacture cycle has generally been as follows:

- Heating of bushes and holding at a temperature between 900 and 930 °C in pit carburizing furnaces fed with methane and carburizing gas,known as endogas, produced in external catalytic generators burning methane. Treatment time is generally between 24 and 36 hours;

- Slow cooling in carburizing furnace to about 600 °C;

- Transfer to suitable chamber for further cooling to about 300 °C;

- Air oooling to room temperature;

- Heating of bushes to 820-860 °C;

- Quenching of bushes in water-oil emulsion;

- Tempering of ends at 550-600 °C in induction furnace and subsequent cooling to obtain a surface hardness of 35-46 HRC;

- Total stress relieving in soaking pit at 180-200 °C for 1-2 hours.

[0010] The complexity of the cycle is evident, particularly the effects on its economy of the treatment times and the four heatings, three of which to high temperature.

[0011] Recent events which caused such a vertiginous rise in energy costs have resulted in a very decided deterioration in the situation. However, it does not appear that any significant changes have occurred so far in the specific field of bush manufacture.

[0012] The object of this invention is to simplify and reduce the cost of making bushes through a.series of measures aimed at simplifying the plant, reduc ing carburizing times, eliminating the heating for quenching and reducing energy oonsumption, particularly as regards methane.

[0013] The present invention provides for the use of techniques that, although actually known, such as high-temperature carburizing with furnace-produced atmosphere and direct quenching, are applied in an original manner to a product such as bushes and incorporated for the first time simultaneously in an optimized treatment cycle.

[0014] The procedure for the manufacture of track bushes as per this invention i_scharacterized by the sequential combination of the following operations:

- Preheating of charge in a treatment chamber of the furnace fed by the combustion gases extracted from the carburizing chamber;

- Transfer of charge to the carburizing chamber fed directly with methane and air only, raising the temperature of the charge to above 900 °C, preferably between 950 and 980 °C, holding at that temperature for the time required to obtain the desired carburizing depth, typically around 10-17 hours for a depth of between 2 and 3 mm;

- Cooling of charge in a cooling chamber of the furnace, bringing the temperature down to between 820 and 850 °C where it is held by means of gases coming from the carburizing chamber;

- Extraction of the bushes one at a time from the cooling chamber by robot and direct quenching;

- Tempering of the ends of the bushes at 550-600 °C, followed by emulsion cooling to obtain the desired hardness of 35-45 HRC and finally total stress relieving in a continuous furnace at 180-200 °C for 1-2 hours utilizing the residual heat of the gases leaving the treatment furnace (carburizing and cooling zones).

[0015] Though this manufacturing cycle certainly utilizes known technologies, they have never been used to make bushes, where the need to save energy and to operate rapidly was felt particularly strongly owing to the low- cost nature of the product.

EXAMPLE

[0016] A series of charges of bushes each weighing 2000 kg, all of the same type of steel, was treated at 950 °C and 980 °C to obtain carburizing depths of between 2 and 3 cm. The treatment times were then varied, while the other parameters remained the same for each change, namely: quench temperature 850°C; tempering of end_sin induction furnace 570 °C, stress relieving for two hours at 180 °C.

[0017] The results obtained can be summarized as follows:

- Carburizing temperature: 950 °C

- Carburizing time (steady state): 12 hours

- Quench temperature: 850°C

- Tempering: 2 hours at 180 °C

- Austenitic grain: 5.5 in centre, 7 in carburized layer (ASTM)

- Structure: martensitic

- Surface hardness: 60 MRC

- Centre hardness: 41 MRC

- Surface hardness of ends: 40 HRC

- Effective depth at 50 HRC: 2.2 mm

- Carburizing temperature: 980°C

- Carburizing time (steady state): 10 hours

- Quench temperature: 850°C

- Tempering: 2 hours at 180 °C

- Austenitic grain: 5 in centre and 7 in carburized layer (ASTM)

- Structure: martensitic in centre; martensitic with residual austenite in carburized zone

- Surface hardness: 61 HRC

- Centre hardness: 42 HRC

- Surface hardness of ends: 41 HRC

- Effective depth at 50 HRC: 2.5. mm

[0018] The ensemble of specific techniques selected - furnace producted carburiz ing atmosphere and direct quenching - and the high recovery of residual heat from the carburizing gas, permits an energy saving in excess of 200 Nm³ of methane per tonne of charge, equal to 40% of an identical treatment using endogas, and this despite the higher carburizing temperature.

[0019] The saving in time is also marked. Indeed, if a carburizing depth of about 2.5 mm is chosen, with the traditional carburizing temperatures between 900 and 930 °C the carburizing time ranges from 20 to 30 hours, which is almost twice that needed with the process as per the present invention.

Claims

1. Process for the production of track bushes for tracked vehicles, from low-carbon boron steel, known in itself, having a mertensitic structure in the centre and on the surface with 5-8 ASTM austenitic grain, centre hardness between 32 and 44 HRC, surface hardness over 55 HRC and hardness of ends between 35 and 46 HRC, characterized by the fact that the bushes, loaded in baskets, are subjected to the follow ing sequential combination of operations:

-preheating of charge in a pretreatment chamber of the furnace fed by combustion gases from the successive carburizing chamber;

-transfer of charge to carburizing chamber, where the carburizing atmo sphere is produced in situ, and where the charge is carburized at a temperature in excess of 900 °C;

-cooling of charge in a third chamber of the furnace, to a temperature of 820-860 °C, maintained by means of combustion gas leaving the carburizing chamber;

-extraction of bushes one at a time by robot and direct individual quenching;

-tempering of ends by known process and emulsion cooling;

-total stress relieving of bushes for 1-2 hours at 180-200 °C in continuous furnace fed by hot waste gases from carburizing furnace.

2. Process as per Claim 1, characterized by the fact that the carburizing temperature is preferably between 950 and 980 °C.

3. Process as per Claim 1, characterized by the fact that the quench temperature of the bushes is between 820 and 860 °C.

4. Process as per Claim 1, characterized by the fact that after direct quenching of the bushes the ends are tempered in an induction furnace at a temperature of 550-600 °C.