EP0079864B1

EP0079864B1 - 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

Info

Publication number: EP0079864B1
Application number: EP82830210A
Authority: EP
Inventors: Walter Grilli; Franco De Meo; Ivan Franchini
Original assignee: Italtractor ITM SpA; ITALTRACTOR MECCANICA ITM SpA
Current assignee: Italtractor ITM SpA
Priority date: 1981-10-23
Filing date: 1982-07-21
Publication date: 1987-11-25
Also published as: IT1171606B; IT8149552A0; EP0079864A1; BR8206000A; DE3277732D1

Description

The 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.
The bush, rotatably coupled to a pin, forms the articulated junction element between the track links.
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 earthmoving machinery operates further increase the stress on the bushes.
The technological properties the bush must have in order to withstand such use conditions include high surface hardness combined with good tensile strength, good fatigue strength and good impact strength. 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 hardnes at the ends of 35-46 HRC.
These requisites are obtainable by using a low-carbon steel (0.13-0.24% C) containing boron (0.0005-0.003% B) with 0.60-0.90% Mn and 0.10-0.30% Si, both forthe 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.
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.
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, although 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 carburizing heat, performed hitherto either on very small parts treated en bloc or even sometimes on large parts, treated singly right from the carburizing 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 treatment 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.
Up to now, as it is stated in Metal Science and Heat Treatment, vol. 21, No 7, 1979, pages 609-611, the bush-manufacture cycles generally comprised a carburizing treatment at a temperature of 960±10°C in carburizing furnaces with natural gas fed directly into the retort.
By using such manufacturing cycles carburizing times last 20-28 hours.
Then, as stated in the same reference cited above, a carburizing in fluidized bed was tested with temperature ranging between 930 and 960°C. It was found that the use of said fluidized bed reduced the carburizing times. However improvements of the inner structure of the bushes were not satisfactory because the bushes were not characterized by a completely martensitic structure, showing small quantities of cementite, austenite and/or perlite.
Such unwanted structures could be avoided by a high complicated treatment involving, after carburization, a slow cooling in carburizing furnace to about 600°C, followed by a transfer to suitable chamber for further cooling to about 300°C; then air cooling to room temperature, followed by heating to 820-860°C, and quenching in water oil emulsion; then tempering of bushes ends at 550-600°C in induction furnace and subsequent cooling to obtain a surface hardness of 35-46 HRC; at last total stress relieving in soaking pit at 180-200°C for 1-2 hours.
The complexity of the cycle is evident, particularly the effects on its economy of the treatment times and the four heatings, three of which at high temperature.
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 occured so far in the specific field of bush manufacture.
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 reducing carburizing times, eliminating the heating for quenching and reducing energy consumption, particularly as regards methane, but maintaining the most suitable internal structure of the bushes.
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.
The process for the production of track bushes for tracked vehicles, from low-carbon boron steel, having a martensitic 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, according to present invention comprises the following steps:

-preheating of bushes in a pretreatment chamber of the furnace fed by combustion gases from the successive carburizing chamber;
-transfer of the track bushes to carburizing chamber, where the carburizing atmosphere is producted in situ, and where the track bushes are carburized at a temperature in excess of 900°C;

-extraction of said bushes one at a time and direct individual quenching;
-tempering of ends of said bushes 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.

The carburizing temperature is preferably between 950 and 980°C.
After direct quenching of the bushes the ends are preferably tempered in an induction furnace at a temperature of 550-600°C.
The manufacture cycle of the invention allows to save energy and to operate rapidly.
The following example illustrates the present invention.

Example

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 ends in induction furnace 570°C, stress relieving for two hours at 180°C.
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 50HRC; 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: 42HRC
-Surface hardness of ends: 41 HRC
-Effective depth at 50 HRC: 2.5. mm

The ensemble of specific techniques seIeεted- furnace producted carburizing atmosphere and direct quenching-and the high recovery of residual heat from the carburizing gas, permits an enetgy 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.
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, having a martensitic 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, comprising the following steps:

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

-transfer of the track bushes to carburizing chamber, where the carburizing atmosphere is produced in situ, and where the track bushes are carburized at a temperature in excess of 900°C;

-cooling 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 said bushes one at a time and direct individual quenching;

-tempering of ends of said bushes and emulsion cooling;

-total strees 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 after direct quenching of the bushes the ends are tempered in an induction furnace at a temperature of 550-600°C.