DE2141175A1 - Spheroidal cast irons - strengthening by thermal and nitriding processes - Google Patents

Abstract

Articles cast from spheroidal graphite cast irons can be strengthened by a thermal treatment consisting of heating to 880-920 degrees C to completely austenitise if possible, followed by ageing or tempering at 600 degrees C. This thermal treatment is then followed by nitriding at 550-600 degrees C using gas liquid or ionic nitriding agents. The material can then be further hardened by shot blasting, or following removal of the nitrided layer, by pressure hardening in areas where most operative stresses are expected. Material treated in this way finds particular application for internal combustion engine parts partic. where high wear resistance and high fatigue resistance is required, e.g. crankshafts cylinder liners, gear wheels, etc.

Description

PROCESS FOR CONSOLIDATING COMPONENTS OF CAST IRON WITH KUGEIGRAPHIT The present invention relates to the manufacture of machine parts, more specifically on processes for solidifying parts made of spheroidal graphite cast iron.

The invention can be used in the manufacture of parts for internal combustion engines, in particular of crankshafts, cylinder liners and gears, camshafts, Washers, valve guide bushings and similar parts are used.

Processes for solidifying parts are generally known Nodular cast iron, which includes the nitriding process.

In the known cases, parts are made of spheroidal graphite cast iron before nitriding, heating to a temperature of 860-950 ° C, holding on this temperature, cooling and then tempering at one. Temperature from 600-740 ° C subject.

The nitriding is usually done at a temperature of 600-6500 C carried out.

The disadvantages of the known methods include the following. The excretion and coagulation of inclusions of the phosphide eutectic at a temperature above 920 ° and lowering as a result of this, the strength and especially the plasticity Incomplete austenitization of the structure at a temperature below 880 ° C, whereby the ferrite content reaches 50% and more, leads to a decrease in strength of the core and to reduce the hardness of the nitrided layer.

At a tempering and nitriding temperature above 6000 C a significant one goes Decay of the eutectoid cementite in front of it, leading to a decrease in strength, a severe distortion of the parts and difficulties in straightening them.

In addition, when the parts are nitrided, the diffusion is below 5500 c made difficult by nitrogen, making it practically impossible to obtain a large Layer thickness (0.5-0.8 mm).

Low and in some cases insufficient fatigue and wear resistance the part arises because of the presence of a large amount in <in the metal structure> the coarse phosphide eutectic of ferrite as a result of austenitizing at temperatures above 9200 C and below 8800 C and the execution of the event at a temperature above 6000 C.

Other disadvantages are: Long production cycle times and large Labor intensity in the manufacture of parts (the cycle of thermal treatment of e.g. waves reaches 200 to 250 hours).

The aim of the invention is to eliminate the aforementioned drawbacks.

The invention is based on the object of a method for solidification of parts made of spheroidal graphite cast iron, by means of which it is possible is, the resistance of the metal to structural changes, the fatigue and To increase contact strength as well as wear resistance, the production cycle too and reduce the amount of work involved in manufacturing the parts.

This task is accomplished with the help of a method of solidification of Parts made of cast iron with spheroidal graphite dissolved, which is the process of nitriding includes, and in which according to the invention the parts of a thermal pretreatment and subjected to work hardening after nitriding.

To achieve the required depth of the diffusion layer and The parts are expediently reduced in the degree of graphitization of the structure nitrided at a temperature of 550-600 ° C.

To ensure a homogeneous structure, precipitation and coagulation of inclusions of the phosphide eutectic turn off the persistence the structure against graphite formation during nitriding and thermal straightening processes to increase and to achieve required high mechanical properties includes the thermal pretreatment the following operations: heating the parts to a Temperature of 880-920 ° C, hold at this temperature until austenitic is achieved Structure of metal, then tempering and (or) aging at a temperature of 580-650 ° C.

In order to increase the fatigue strength additionally, the nitrided Parts subjected to hardening by shot peening.

To the fatigue strength and resistance of the parts to one-off increasing dynamic overloads, reducing the nitriding cycle and straightening To simplify the work hardening by pressure hardening after lifting off the nitrided layer In the following, the present invention will be carried out through detailed description and working examples explained.

The proposed method is that the parts are made of cast iron with spheroidal graphite gas, liquid or ion nitriding at one temperature from 550-580 ° C. In the case of cast iron, which is alloyed by Cr, Ni, Mo and Cu and compared to unalloyed cast iron higher resistance of the structure against graphitization, the nitriding temperature can be around 20-300 c 'can be increased. The nitriding time and, accordingly, the technological depth The nitriding layer should ensure that a finished part has a hardness of HRC # 40 is received taking into account the metal decrease due to delay.

The technological depth of the nitriding layer for parts of the type Crankshafts and cylinder liners is practically in a range from 0.3 to 0.8 mm.

The thermal pretreatment of the parts includes the following operations: Heat the parts to a temperature of 880-920 ° C, hold at this temperature until the austenitic structure is achieved in the metal, then cooling, tempering and (or) aging at a temperature of 580-650 ° C.

The procedure for the thermal pretreatment is dependent on the structure and degree of graphitization of cast iron in the (cast) initial state as well as the chemical composition of the cast iron. The more in the structure Ferrite is present and the higher the degree of graphitization of cast iron in its initial state is, the higher the austenitizing temperature and the longer the holding time. Around the appearance of graphitization during cooling after austenitization to eliminate, the parts to be processed are in O1 (hardening) or in air Forced circulation (normalization ) cooled, When the austenitizing temperature is increased over 9200 C goes beyond the limits and joints of the eutectic grains the excretion and coagulation of inclusions of the phosphide eutectic (eutectic Grain boundary deposition), the drop in strength and especially plasticity (specific Elongation shows values below 1.5%). At a temperature below 8800 C the austenitization of the structure is incomplete1 and the ferrite content in the structure after cooling it reaches 35% and more.

In unalloyed cast iron with a ferrite content of 60-85% the breaking strength of cast iron 48-52 kp / mm2 which is about 30% less than in the pearlitic structure with 7-15% ferrite (fruit strength 65-75 kp / mm²).

Slow cooling of the parts after austenitizing (for example the cooling of large crankshafts on the furnace hearth without forced circulation) is extreme undesirable, since in this case a noticeable graphitization dea eutectoid in cast iron Cementite observed from the increase in the amount of ferrite and the decrease the strength (by 20-35%) is accompanied and the intense graphitization of the structure with subsequent tempering, aging, nitriding and thermal straightening processes evokes.

The tempering temperature of the parts will depend on the degree of graphitization the structure obtained after cooling from the austenitizing temperature was, and from the chemical composition of cast iron. For plain cast iron, the tempering temperature will be in a range of 580-6000C set. At a higher tempering temperature (especially in the normalized state) finds a noticeable graphitization of the cast iron and a drop in strength instead of.

The process intensifies with subsequent aging, nitration and thermal straightening processes.

For alloyed cast iron (cast iron with 0.5-1% Cu, Ni + Mo; Ni + Mo + Cu; Cr + Ni + Ko + Cu etc.) the tempering temperature is expediently up to 630-650 ° C increased. In this case, in addition to the high mechanical properties the elimination of residual stresses and minimal distortion of the parts Nitriding ensured. For example, increasing the tempering temperature allowed the Cylinder liners made of spheroidal graphite cast iron, alloyed by 1-1.5% Ni, 0.3-0.5% Mo, from 580-600 ° C up to 630-6500 C, the rejection of the liners due to delay to eliminate where changing the inside diameter in a number of Cases reached 1-2 mm.

For unalloyed cast iron, which is characterized by a low resistance of the structure against graphitization, it is not advisable to start the tempering process to be carried out.

This recommendation applies primarily to those to be nitrided Parts made of spheroidal graphite cast iron, to which the requirements of a high contact and fatigue strength (cock gears, vortex shafts).

The aging of the parts is postponed at a temperature of 580-6500C Reduction of the cut allowance made in the amount of 50-90%. The duration of aging is 5-12 hours. The ablation speed after aging should be in one Range from 40-1000 / hour lie.

In some cases, the initial state of cast iron is through high Resistance to graphitization marked @ This is the case with castings made of alloyed cast iron or, in the case of castings, after crystallization has ended e.g. by knocking the castings out of the molds at a temperature of 900-850 ° C accelerated cooling. In these cases thermal pretreatment can be used of cast iron consist of tempering alone prior to nitriding. If e.g. the fatigue strength normalization, tempering, aging, nitriding and thermal Cast iron exposed to straightening processes with a well-developed graphitization of the Structure in the final state (ferrite content 50%) is rated for 100%, so is with one prior to nitriding only subjected to tempering and also containing 50% ferrite, but no noticeable graphitization showing cast iron the creep strength 20% higher (for example, the fatigue strength of the crankshaft of the diesel locomotive engine is marked by stresses in grooves of 2500 kp / mm2 or 3100 kp / am2).

The thermal pretreatment, the appearance of graphitization and the initial structure influences the hardness and wear resistance of the nitrided layer.

With the ferritic cast iron structure of the crankshafts of diesel locomotive engines it is almost impossible to find a hardness when gas nitriding in furnaces with 5 m3 and more muffle space of the layer above HRC = 36 - 40, while with a pearlitic structure the The hardness of the layer is HRC = 45-50 when tested on a friction machine of nitrided crankshaft samples made of cast iron with spheroidal graphite and a hardness of HRC = 38-40 in a pair with an insert piece with an anti-friction layer made of lead bronze or aluminum alloy is the wear resistance of the shaft with pearlitic The structure of the core is the same as that of a shaft with the structure of ferrite + 10% pearlite. However, the contact strength of nitrided cast iron with spheroidal graphite is pearlitic Structure 50-60% higher than that of cast iron with a ferritic structure of the core.

The thermal pretreatment also has an effect on the warpage of parts during nitriding. The less the cast iron structure against the graphitization is stable and the stronger this is during nitriding, the greater the changes in volume and preforming of the part as a result of the structural transformations and consequently also the warpage of the part, since in this case the one resulting from the structural transformations of the core Deformation adds up to the deformation caused by the formation of the diffusion layer in the surface layers of the metal.

The sections are subjected to work hardening by shot peening the nitrided parts of nodular cast iron subjected to the largest Experiencing tension at work (grooves in the crankshafts, tooth gaps in the gears i.a.).

The shot peen hardening is done by means of steel balls 0.6-2.0 mm with a Pressure of 4-5 atm for 0.5-1 min with a distance between nozzle and part of 150 to 300 mm. This shot peen hardening results in the following Characteristic curve of the residual stresses in the depth of the nitrided layer: in one depth from 0.02-0.04 mm the remaining compressive stresses are equal to 65-80 kp / mm2 then gradually to zero at the limit of the nitriding layer with the like (without peen hardening, the residual stresses have a more complicated picture: for example, with a nitrided layer 0.7 min thick at a depth of 0.02-0.04 mm on the characteristic curve of the remaining stresses the first maximum with a compressive stress value observed up to 40-50 kp / mm²; at a depth of 0.1-0.2 mm are tensile stresses of 0 to 8 kp / mm² recorded; then the second maximum corresponds to the magnitude of the compressive stresses up to 15-25 kp / mm² to be noted). So the shot peen hardening increases the compressive stresses in the entire depth of the nitrided layer. The result is the creep strength of nitrided cast iron by shot peen hardening additionally un 20-45% increased.

Thanks to the increase in fatigue strength (fatigue strength) it becomes possible to simplify the thermal pretreatment (e.g. normalization and hardening with aging process), the degree of security and consequently also to increase the reliability and useful life of the part and in some cases for the production of parts made of spheroidal graphite cast iron instead of alloy steel (e.g. for the production of nitrided crankshafts for diesel @ okmotines instead of the alloy steel shafts).

The work hardening by pressure hardening is carried out after the acceptance the nitrided layer through. The nitride layer is lifted off and then the metal subject to work-hardening on the sections of the part that are in constructive The most stressed aspects (e.g. crankshaft grooves). The nitrided layer is completely removed in the grooves by mechanical processing, whereupon the Throats are subjected to pressure hardening by rolling. Here you can see in front of the Nitriding an addition in the grooves, which is chosen so that between the grooves and nitriding layer on the peg a separation zone 1-2 mm wide and between the fillet and crank web - up to 5 mm wide remains.

The addition for mechanical processing according to the depth should be the thickness the nitrided layer by 30-70%. The separation zones between the nitrided layer and the plastic deformed zone of the throat prevent the Nitriding layer is damaged during pressure hardening and the tensile stresses which at the edges of the nitrided layer in the zone of maximum stress concentration (in the grooves) affect the fatigue strength of the shaft.

With such a technology of manufacturing and solidifying Parts made of spheroidal graphite cast iron make it possible to simplify the straightening of the parts, a higher fatigue strength than a nitrided part and below that too the resistance to one-time dynamic overloads while maintaining a ensure high wear resistance of the surfaces of the parts subject to wear and tear stressed (for example the cylindrical part of crankshaft journals).

The question of distortion of nitrided parts is the main one. It it is impossible to completely avoid the distortion of parts, which is why it is to be removed of the warpage, a cutting allowance is provided and straightening is used. E.g. -The cutting allowance for crankshafts reaches 0.3 mm for each side at the depth the layer 0.7mm while it is for. Cylinder liners 0.2 mm with a layer depth of 6 mm amounts to. The striking of nitrided crankshafts made of spheroidal graphite cast iron, the length of which is 2000-2500 mm, reaches 0.5-2 mm. Such crank wool will be the thermal straightening in the furnace under load or without load at a temperature of Subjected to 500-550 ° C. the Method is imperfect because of this a partial softening of a number of flutes takes place, the nitrided Due to their high elasticity, waves are difficult to straighten.

The number of straightening operations has been reached. 3-5. It doesn't succeed Warping and straightening of the shafts even as they rotate during nitriding to avoid. In connection with this, the use of liquid nitriding (e.g. of the Tenifer process) for the consolidation of complex shapes and lengths Parts excluded because their warpage is greater than the depth of the layer. Be it also noted that a major disadvantage of the manufacturing technology of nitrided Parts that are warped consists in that of the surface of the part being the hardest and consequently the most wear-resistant part of the diffusion layer is lifted off.

The use of pressure hardening with a previous decrease in the nitriding layer e.g. with crankshafts it allows: - to the imperfect thermal straightening of the Foregoing waves and uidae the time required to straighten a wave Reduce 10-15 times su; - increasing the fatigue strength of the. Wave opposite ensure a shaft with nitrided fillets by 30-50%; - the nitriding temperature (for alloyed cast iron) to increase by 20-400 G and by twice and even more the to reduce the technological depth of the nitriding layer (for example is for cylindrical parts of the shaft journal 0.3 mm instead of 0.7-0.8 mm is sufficient), and thereby reducing the nitriding cycle by 1.5-4 times (currently achieved the cycle of gas nitriding of e.g. cylinder liners and crankshafts made of cast iron with spheroidal graphite 110-150 hours); - to reduce the take-off considerably and on thanks to the grinding of cylindrical parts of the shaft journals after nitriding Eliminating shaft distortion by means of pressure hardening of fillets entirely; - to maintain the hardest surface layer and, as a result, an increase the wear resistance and the increase in the service life of the shaft by 1t5-2 times to ensure; - Equivalent key data on fatigue and wear resistance of nitrided shafts made of spheroidal graphite cast iron with nitrided shafts of alloyed To achieve steel.

The proposed method of consolidating the parts allowed S; the production technology of ton nitrided parts made of cast iron iit spheroidal graphite to significantly simplify and the operational safety and service life of parts made of spheroidal graphite cast iron. Applying this procedure to solidify the crankshafts of diesel locomotive engines has enabled a on the Manufacture of nitrided shafts from alloyed steel to dispense and their cost stwa by 65%. Compared to the non-nitrided wool made of cast iron with spheroidal graphite assign the crankshafts manufactured according to the proposed method a wear resistance that is more than 2.5 times higher, the wear resistance of nitrided cylinder liners made of spheroidal graphite cast iron is five times higher than that of the liners made of alloyed gray cast iron while the strength 1.5 - 2 times is higher.

It should be particularly emphasized that at the points of stress concentration of nitrided parts made of spheroidal graphite cast iron, the use of work hardening by pressure hardening after removing the nitrided layer at these points during use made possible by carbide-tipped chisels.

For example, the nitriding layer was in the throats of a crankshaft made of spheroidal graphite cast iron using a carbide-tipped chisel at speed the shaft 12 rpm and the chisel feed rate 0.2-0.4 mm / revolution.

The known methods (for example tinning, brushing with Water glass) for nitriding protection of sections in which the nitriding layer is to be carried out the pressure hardening is lifted, are expediently not used "because these methods are labor-intensive and there is no clear dividing line between the nitrided layer and the pressure-hardened layer Ensure metal.

The above-mentioned protective methods against nitriding are particularly difficult on large and complex parts (e.g. crankshafts of diesel locomotors) to realize.

Therefore it seems more appropriate to remove the nitriding layer from the parts to be lifted off only at the points of stress concentration and then only at these points subject to strain hardening by pressure hardening.

Claims (5)

P A T E N T A N S P R Ü C H E: 1. Process for strengthening parts made of spheroidal graphite cast iron, which includes the process of nitriding, d a u r e h e k en n z e i c h n e t that the parts undergo a thermal pretreatment and then after nitriding be subjected to the consolidation of validity. 2. The method according to claim 1, d a d u r c h g e k e n n -z e i c Not that the nitriding is carried out at a temperature of 550 ... 600 ° C will. 3. The method according to claim 1, d a d u r c h g e k e n n -z e i c It should be noted that the thermal pretreatment includes the following operations: heating of the parts to a temperature of 880-920 ° C, hold at this temperature until Achieving the austenitic structure, then tempering and (or) aging in one Temperature from 580 ... 650 ° C. 4. The method according to claim 2 or 3, d a d u r c h g e -k e n n z E i n e t that the work hardening is carried out by shot peening. 5. The method according to claim 2 or 3, d a d u r c h g e -k e n n z e i c h n e t that work hardening occurs at the points of stress concentration is carried out by pressure hardening after lifting off the nitrided layer.