US2895859A - Process for the manufacture of piston rings - Google Patents

Description

United States PatentQ PROCESS FOR THE MANUFACTURE or PISTON RINGS Lucien Pras, Billancourt, France, assignor to Regie Nationale des Usines Renault, Billan'court, France No Drawing. Application June 30, 1955' Serial No. 519,220

Claims priority, application France August 4, 1954 The invention relates to piston rings for internal combustion engines and relates to a process for the manufacture of said-piston rings enabling a satisfactory sealing of the combustion chamber to be eifected and the wear on said rings to be reduced.

The piston rings of explosion engines or internal combustion engines are currently manufactured by stack moulding in green sand from a cast iron, the average composition of which is: C=3.50%; Si=3.0%; Mn=0.60%; P=0.40%; S=0.05%.

The aim, in this usual manufacture, is for the cast metal to consist of graphite, pearlite and possiblysteadite (phosphorus eutectic Fe-C-P). Every elfort is made to avoid cementite, the presence of which would render any machining impossible, and ferrite, a soft constituent, the presence of which is a cause of wear.

The imperative requirements of machining lead to working rules in the foundry which reliably avoid cementite, but in these circumstances it is difiicult to avoid ferrite. Slight variations in the casting conditions cause noticeable variations in the proportion of the latter constituent, so that constant supervision is necessary to ensure constancy in the quality of such a product.

The piston rings thus produced have long proved satisfactory in use. They are becoming inadequate, however, even when the desired structure of fine graphite Patented July 21, 1959 carbon capable of being dissolved at the temperature in question, to be so. A temperature between 760 and 920C. is suitable. This heating is followed by quenching, preferably in oil, thus conferring a martensitic structure to the metal. It is also possible to efiect a step quenching comprising homogenization at a temperature and pearlite is obtained, in the face of the-increased requirements of modern e ngine's' 'Which are subjected to r he loads ndrc atioaa zspe s. .1 1

When conventional piston rings are used in the most modern engines, of cars for example, sealing defects and premature wear appear, which cause a reduction in power and an excessive consumption of lubricating oil.

An important progress is-achieved by the present invention, which relates to a method of manufacturing piston rings which makes it possible to achieve an excellentsealing of the combustion chamber, said sealing beingmaintained during ,long use of the engine, so that the need for dismantling never arises by reason of'deterioration of the piston rings.

According to the invention, the piston rings are obtained by cast iron moulding, the composition of the iron being comprised within the following limits:

Te 0.000 to 0.050%

After casting, the parts, which are entirely white in structure, are subjected to a treatment which comprises the following sequence of operations:

(a) Reheating to a temperature at which austenitization takes place and for a time suflicient for all the very slightly above the beginning of the martensitic transformation and short enough in duration to prevent the beginning of any bainitic transformation, then subsequent cooling to produce the martensitic transformation.

(b) A second heating, to bring forth nucleation of the graphite, at a temperature comprised between 420 and 550 C. for a time comprised between 10 minutes and 3, hours.

(c) A third heating, up to a temperature greater than the temperature at the end of the eutectoid transformation, and maintenance at this temperature for the time sufiicient and necessary for the carbon in the primary cementite to be entirely graphitized. This heating is followed by cooling, either in still air or in oil, depending on the final hardness desired.

(d) Temperingat a suitable temperature to ensure the desired degree of hardness. For example, when graphitization has been followed by oil quenching, the tempering will be eifected at:

600 C. for a desired final hardness of 105 RB 560 C. for a desired final hardness of 35 RC 520 C. for a desired final hardness of 38 RC (e) After these thermal operations, the piston rings are machined and split. A suitable tension is conferred on them by relieving the stresses set up while the ends of the ring are suitably parted. The stress relief is obtained by heating at a temperature slightly less than the tempering temperature or by any other equivalent means.

The piston rings thus obtained are characterized metallographically:

By the absence of primary cementite;

By a distribution of the temper carbon in very fine, very numerous and very regular granules, e.g.' 6,000 granulesof graphite for mmP;

By a matrix having'a very fine sorbitic-or pearlitic structure;

By a modulus of elasticity greater by about 50% than that'of ordinary grey cast iron. For example, the modulus of elasticity of the piston rings accordingto the invention has been found to be 18,000 kg./m.m. whereas the modulus of the conventional piston rings of grey cast iron does not exceed 13,000 kg./mm.

By ahigher tensile strength and hardness than those of theusual piston-ring metals. For example, in the pistonrings according to the invention, a tensile strength of kg./mm.'- and a breaking load of kg./mm. are found;

By a remarkable absence of brittleness such as is not found in the usual piston-ring metals;

By a very low coefiicient of friction.

A specific example of manufacture of piston rings according to the invention is given below.

After stack casting in green sand, piston rings of cast iron having the composition:

0 Si Mn Cu P S Nucleation heating for 2 hours at 450 C.;

Graphitization heating for 1 hour at 890 6., thencooling in still air;

.Tempering at 600 C., thu s obtaining ahardness of from 103 to 104 RB (measured by a ball in the Rockwell machine, load 100 kg); 7

Machining, and keeping the ends at the required distance during heating for 5 to 7 minutes at'580 (5.;

Grinding to the final dimensions.

The piston rings manufactured as described above ensure a very long period of service. They are particu larly valuable in engines in which the cylinder linings are of hard cast iron. In such engines, the. wear of the new piston rings is much less (by about30%) than that of piston rings of conventional manufactureunder the same conditions of use. The breakages in se viee, which occur fairly frequently in very cut-awayscraper-rings currently manufactured, never occur with the new piston rings.

I claim:

1. The process of malging alloyed castpiston rings which are substantially free" of primary cefnentite and brittleness, and have a low coeflicient of friction, comprising stack molding in a green sand mold the piston rings of a cast iron having the composition 3.004.00.% carbon, 1.503.10% silicon, 0.6-1.5% manganese, 0.8-2.0% copper, 0.05O.70% phosphorus, .02-0. l% sulphur, and the balance substantially all iron, whereby the cast piston rings have a white structure, heating to a temperature slightly above that of the eutectoid trans formation point so that austenitization takes place and for a time suflicient for all the carbon to be dissolved, quenching while maintaining a substantially martensitic structure, again heating at a constant temperature. between 420 C. and 550 C. for a period of the order of three hours to bring forth graphite nucleation, inereasing the temperature after said nucleation heating to a temperature higher than the eutectoid transformation point for a period of time sufiicient to graphitize the carbon in the primary cementite, cooling at a controlled rate, said cooling rate being a function of the final hardness, to be obtained, and tempering at a predetermined temperature to insure a predetermined hardness.

2. The process of making alloyed cast piston rings which are substantially free of primary cementite and brittleness, and have a low coeflicient of frietion, comprising stack molding in a green sand mold the piston r n of a cast ro h in th om si iq 3.004.00% carbon 0.054070% phosphorus 1.so-3.10% silicon 0.024115% sulphur 0.6-1.5% manganese 00.05% telluriu n 0.8-2.0% copper and the balance substantially all iron, whereby the east piston rings have a white structure, heating to a temperature slightly above that of the euteetoid transformation point so that austenitization takes place and fear a time sufiicient for all the carbon to be dissolved, quenchingin oil While maintaining a substantially martensitic structure, again heating at a constant temperature between 420 C. and 550 C. for a period of the order of 3 hours to bring forth graphite nucleation, increasing the temperature after said nucleoli germination heating to a temperature higher. than the eutectoid transformation point for a period of time sufiicient to totally graphitize the carbon in the primary cementite, cooling at a controlled rate, said cooling rate being a function of the final hardness to be obtained, tempering at a predetermined temperature to ensure a predetermined hardness, machining the rings and cutting a gap therein to split the rings, stress relieving the rings at a predetermined temperature with the split being set at a predetermined gap opening, said stgess relieving temperature being slightly less than the tempering temperature.

3. Th Process of making al oyed ast piston r n whisk: are bs n iall fir s 9! rima y cemen t an bri tl n ss. an ha a l w oefli s t of i tion, inpri ins stack meldin in a e n nd mold h pis on rings of a cast iron having the composition 3.54% carbon 1.86% silicon 1.24% manganese 1.29% copper 0.44% phosphorus 0.04% sulphur and the balance substantially all iron, whereby the cast piston rings have a white structure, heating to a temperature of 810 C. and maintaining this temperature for substantially 30 minutes so that austenitization takes place and for all the carbon to be dissolved, quenching in oil while maintaining a substantially marteiisitic stiucture, again heating at a constant temperature of 450 C. for a period of two hours to enhance graphite nucleation, graphitization heating at a temperature of 890 C. for 1 hour, cooling in still air at a predetermined rate necessary to obtain a predetermined hardness, tempering at substant ally 600 C. for a period of time to obtain a hardness of between 103 and 104 RB when measured by a ball type Rockwell machine with a load of kg, machining the rings and cutting a gap therein to split the rings, stress relieving the rings at a temperature of substantially 580 for a period of between 5 to 7 minutes with the split in the rings being set at a predetermined gap opening, and grinding to final dimensions.

Refie -ences Cited in the tile of this patent S E A E S 2. 85. 9 Hul sr n ---..-7--.-..-- a 2, 1 40 ,650 B nc f -=1 Sep 0. 0 2331.88 i g hol t- 1 1943

Claims (1)

1. THE PROCESS OF MAKING ALLOYED CAST PISTON RINGS WHICH ARE SUBSTANTIALLY FREE OF PRIMARY CEMENTITE AND STACK MOLDING IN A GREEN SAND MOLD THE PISTON RINGS OF A CAST IRON HAVING THE COMPOSITION 3.00-4.00% CARBON 1.50-3.10%SILICON,0.6-15% MANGANESE, 0.8-2.0%COPPER, 0.05-0.70% PHHOSPHOURS,2.02-0.15% SULPHUR,AND THE BALANCE SUBSTANTAILLY ALL IRON WHEREBY THE CAST PISTON RING HAVE A WHITE STRUCTURE HEATING TO A TEMPERATURE SLIGHTLY ABOVE THAT OF THE EUTECTOID TANSFORMATION POINT SO THAT AUSTENITIZATION TAKES PLACE AND FOR A TIME SUFFICIENT FOR ALL THE CARBON TO BE DISSOLVED QUENCHING WHILE MAINTAINING A SUBSTANSTIALLY MARTENSITIC STRUCTURE AGAIN HEATING AT A CONSTANT TEMPERATURE BETWEEN 420*C.AND 550*C. FOR A PERIOD OF THE ORDER OF THREE HOUR TO BRING FORTH GRAPHITE NUCLEATION INCREASING THE TEMPERATURE AFTER SAID NUCLEATION HEATING TO A TEMPERATURE TURE HIGHER THAN THE EUTECTOID TRANSFORMATION POINT FOR A PERIOD OF TIME SUFFICENT TO GRAPHITZE THE CARBON IN THE PRIMARY CEMENTITE COOLING AT A CONTROLLED RATE SAID COOLING RATE BEING TEMPERING AT A PREDETERMINED TEMPERATURE TO INSURE A PREDETERMINED HARDNESS.