DE3912916A1 - Process for producing a casting from lamellar-graphite cast iron and camshaft produced thereby - Google Patents

Abstract

The invention relates to a process for producing lamellar-graphite cast iron for at least partially remelt-hardened castings. The invention is characterised in that the cast iron is alloyed with (in % by mass) 2.8 to 3.25% of carbon 0.1 to 0.4% of chromium 0.3 to 0.5% of copper at most 0.15% of sulphur [1.7(%S) + 0.2] to [1.7(%S) + 0.3] % of manganese a silicon content within the range from 1.8 to 2.5% in a quantity according to the relationship %Si = 0.334. 2ROOT (7.5.(%C)-PG-16.5)<2>+30.PG.(%C) + +0.334.PG-2.48.(%C)-0.01.M+O.615.(%Cr) -0.27.(%Cu)+5.6+/-0.1 with the graphitisation potential of PG = 0.657M<-0.65>, and M (modulus in cm) being the quotient of the volume fraction of the heat-bearing surface of the casting. <IMAGE>

Description

The invention relates to a method for producing a Cast body made of cast iron with lamellar graphite, the Surface are at least partially remelt hardened and a camshaft manufactured in this way.

The production of castings subject to wear, in particular from camshafts for internal combustion engines Cast iron with lamellar graphite with by local remelting built up ledeburitic boundary layers is the state of the Technology. For example, camshafts are made from one Gray cast iron with a carbon content of about 3%, one Silicon content of about 1.7% and a manganese content of about 0.7% to form a fine lamellar graphite optionally alloyed with titanium in the order of 0.1% (Technical reports Metallurgical Practice 18, 1980, No. 12, Pp. 1115-1127). The camshafts are remelted on an industrial scale by using an electric arc Tungsten electrode in an argon atmosphere (TIG process), see DE-PS 27 03 469. Procedures with other power sources (lasers, Electron beam, s. e.g. B. DE-OS 22 44 220) are such. Currently too expensive or still at the stage of development. The burner is in a line movement over the surface of the patient to be treated Casting led. With camshafts z. B. you get the areal expansion of the remelting zone in that one  Line after line in a circumferential movement around the around the hardening cam or by turning the burner reverses across the circumference of the cam, while the shaft a slow rotation. The hub of the Reversing movement corresponds approximately to the width of the cams. The The position of the burner axis is expedient against the Direction of rotation of the shaft and inclined to the vertical so that gravity always counteracts the molten material the cold one and not against the one that was already warmed up Area presses and the high achieved in this way Solidification rate prevents drainage.

The surface of the originally gray solidified, i.e. H. at of eutectic solidification exclusively graphite and The workpiece containing austenite is thus added in portions remelted, due to the rapid heat emission the individual portions of the cold core material Melt white, d. H. in eutectic crystallization solidify to form Fe₃C and austenite. To this In this way, a layer of layers is created remelting layer consisting of ledeburitic structure.

FIG. 1 is a metallographic micrograph each Umschmelzlagen 1, but only clearly visible when it comes between them under special conditions as unacceptable flaws appearance to graphite precipitates second An originally austenitic zone 4 is arranged between the cold core 3 and the remelting layers 1 . The critical cooling rate of this austenite is so low because of its comparatively high carbon content that after the ledeburitic surface layer has solidified, martensite easily forms there. The volume of this martensite is higher than the volume of the ledeburite, so that cracks are formed and the surface layer can even be blasted (flaking).

By preheating the workpiece to approx. 673 K can Martensite formation are largely suppressed. Besides, will by preheating the thickness of the ledeburitic surface layer favorably influenced.

The chemical composition of cast iron is generally described in sub-eutectic range of the Fe-C-Si system chosen so that in the cast state next to pearlitic converted primary austenite and eutectic austenite only eutectic graphite and none eutectic cementite is present in the structure. The latter is over Avoid for reasons of good machinability.

The crescent-shaped graphite inclusions 2 in FIG. 1 between two ledeburite layers are secondary, that is to say graphite resulting from the partial fall of the first layer as a result of heating when the second layer is melted. After machining the workpiece, they can reach the surface and then cause excessive wear. The punctiform graphite inclusions 5 likewise to be observed within the layers are less critical in this respect, but are also undesirable. Another inadmissible defect is small gas bubbles 6 inside the remelted layers 1 . A workpiece with such bubbles is unusable for use. A further defect is melting 7 in Fig. 1 (material deficit), which can arise from the drainage of iron when the overheating above the liquidus temperature, ie the distance to the solidification temperature, is too high.

The said graphite precipitates occur when otherwise the remelting conditions are the same as the thermodynamic activity the carbon is too high, d. H. Carbon from the Solution is displaced. The elimination process is additional by kinetic factors, e.g. B. the preheating temperature Remelting affected. At high temperatures Sickle graphite formation favored and complicated by low. The setting of any low preheating temperature is however, not permissible because this means the share of unwanted martensite increases.

The thermodynamic activity of carbon in cast iron is in the order of falling effect by C, Si, Al, (Cu) and Ni increased. The effect of silicon is solid Condition apparently stronger than in the liquid. It is lowered in the order of increasing effects by (Cu), Mn, Cr, V and Ti. Cu behaves ambivalently in that it solidifies during solidification Graphite formation, but in the solid state the cementite formation favored.

Significant influences on graphite formation during solidification are kinetic in nature: carbon crystallizes from the Only melt in the form of graphite without problems if the Solidification rate not too high, i. H. the wall thickness of the casting or its module (ratio of the casting volume to its free surface) are not too small as well when, in addition to sufficiently high thermodynamic Carbon activity also has enough solid germs. Usually serve as germs from a deoxidation of the Cast iron resulting SiO₂ particles.  

Cast iron with lamellar graphite normally contains approx. 40 to 80 ppm oxygen. However, values down to 20 and very rarely up to 200 ppm are also observed. As can be seen from oxide spectra ( FIG. 2), the oxygen is present in a bound form, with the exception of a small amount of the so-called surface oxygen (surface A) , which is also molecularly deposited on the surface of graphite lamellae, namely as MnO (surface B), FeO ( Area C), SiO₂ (area D), silicates (area E) and as higher-quality oxides (area F), mainly Al₂O₃.

The bubbles formed during the melting process come from a Reduction of some of these oxides with the carbon of the melt according to the general equation MeO + C = Me + CO. First Oxides with the smallest educational energy become a line reduced. These are FeO, MnO and under certain conditions also SiO₂.

Practice has shown that when remelting Castings, such as camshafts, from various sources Errors that occur can be divided into two main groups:

• - Well deoxidized camshafts were always free of bubbles, but often had graphite deposits,
• - Badly deoxidized camshafts hardly showed Graphite excretions, but there are more bubbles.

The object of the present invention is to create a Process for the production of castings, in particular Camshafts for remelt hardening by adjusting one appropriate chemical composition using appropriate Alloy elements with the lowest possible cost in such a way that none of the described error occurs and in particular a remnant graphite-free remelting layer is created.

According to the invention, this object is achieved in such a way that that the cast iron with (in mass%)

2.8 to 3.25% carbon
0.1 to 0.4% chromium
0.3 to 0.5% copper
at most 0.15% sulfur
[1.7 (% S) + 0.2] to [1.7 (% S) + 0.3]% manganese
Silicon within a range of 1.8 to 2.5% in an amount according to the relationship

% Si = 0.334 * +
+0.334 P · _G · -2.48 (% C) -0.01 · M · + 0.615 (% Cr)
-0.27 · (% Cu) + 5.6 ± 0.1

with the graphitization potential P _G = 0.657M ^-0.65 , and
M (module in cm), this is the quotient of the volume fraction of the heat-conducting surface of the casting,

is alloyed. The iron also contains the usual ones unavoidable impurities.  

In the case of commercially available camshafts, the graphitization potential PG is 0.9 to 1.1 and the module M is 0.4 to 0.6.

The invention solves the problem by defining graphitization adapted to the geometry of the casting potential such that to achieve the cast state required graphitic, d. H. ledeburit-free structure smallest required constant C activity set becomes. The kinetic contribution to graphite crystallization (heterogeneous nucleation catalysis) does a suitable one Deoxidation process. This is the most important requirement for the freedom from secondary graphite after remelting Fulfills. By adding suitable alloying elements in suitable concentrations should the C-activity in the solid Condition then continues to safely avoid secondary graphite be reduced. The formation of bubbles is avoided by sufficient stability of the oxides is ensured. The Meltdowns are caused by limiting the liquidus temperature avoided to a minimum. In the following, the Effects of the individual influencing factors according to the invention to be discribed:

So that there is no material deficit when the remelting process Melt occurs, the liquidus temperature is min. 1200 ° C start. This results according to the equations

TL = 1653-119.CEL

CEL = C + 0.25.Si + 0.5.P

the largest permissible CEL value of 3.80 as a criterion for determining% C _max and% Si _max .

The criterion for% C _min and% Si _min results from the necessity that a sufficient amount of eutectic is available for the success of remelt hardening, ie conversion of graphite-austenite into the austenite-cementite eutectic (ledeburite). It must not be significantly lower than that which corresponds to the upper limit of the CEL value at 3.8 (approx. 80% eutectic) and was approx. 65% according to a lower limit of 3.5 with the CEL value.

The reducibility of SiO₂ and thus the formation of bubbles depends on the position of their equilibrium temperature according to K.E. Höner and S. Baliktay (Giess.-Forsch. 25, 1973, No. 1, pp. 21-27) and H. Schenk and E. Steinmetz (impact parameters of Accompanying elements of liquid iron solutions and their mutual Relationships. Verl. Steel iron. Düsseldorf 1968)

from. It increases with increasing Si and decreasing C content. If the remelting temperature is equal to T _G , the reduction takes place according to the equation

SiO₂ + 2 C = Si + 2 CO

but not for kinetic reasons. This is only happening at some distance from balance. At a given the melting temperature, this distance increases, or the Driving force of the reduction reaction with falling Equilibrium temperature.  

The lowest permissible T _G position to avoid the SiO₂ reduction and thus the formation of bubbles when remelting was set at 1380 ° C. The first criterion for Si _min and another for C _{max are} thus known.

The above conditions are met at the Definition of the C range from 2.81 to 3.25% and the Si contents from 1.9 to 2.5%.

Chromium in an amount of 0.1 to 0.4% reduces the coals Solid substance activity to avoid secondary graphite. The copper offers 0.3 to 0.5% Possibility to limit the amount of manganese, thereby Manganese increases on the grain boundaries, which increase the risk of Significantly increase the formation of martensite, largely avoided the pearlite structure in the same way as with Manganese is stabilized.

Silicon increases carbon activity in the solid state apparently stronger than in the liquid. This can be influenced by further measures can be compensated, so by a limitation the aluminum content to 0.004 to 0.009%, an Al₂O₃ content of at least 0.0005% and / or by adding 0.03 to 0.06% Ti and / or 0.03 to 0.08% V.

Titanium and vanadium reduce the carbon activity more than chrome and manganese, but they also have a higher one Affinity for oxygen and nitrogen. Your concentrations were made taking into account the cast iron Oxygen and nitrogen potentials of the solubility products,  the activity coefficients and their changes over time the dissolution of alloy additives so chosen that titanium Proportionally, nitrides can hardly form oxides, Vanadium, however, due to the shielding effect of titanium practically complete for lowering carbon activity is available.

To reduce as much as possible when pouring in air Cast iron must be practically unavoidable and more stable during the casting process with the aim of formation Oxides are deoxidized, as a result of which a partial Slagging of these oxides the total oxygen content of the Cast iron decreases more or less. By remelting Experiments with camshafts have been shown to cause bubbles then certainly not form if the equilibrium temperature of SiO₂ reduction with carbon Melt is 1380 ° C and the ratio of SiO₂ to that as FeO + MnO bound oxygen the value of 2.0 exceeds. In such conditions, others have iron existing gases are of no importance.

The following examples illustrate the invention:

For a test camshaft with the module 0.48 cm and the required graphitization potential PG = 1.05, the chemical composition was 3.0 to 3.1% C, 2.0 to 2.1% Si, 0.3 to 0.4% Cr and 0.4 to 0.5% Cu prescribed. At 3.1% C, 2.05% Si, 0.4% Cr and 0.5% Cu the structure was mottled in the as-cast state, ie it consisted of both eutectic graphite and ledeburite. For this reason, the camshaft was practically not machinable. However, when the Si content was increased to 2.2% in accordance with the formula shown in claim 1, the structure was free of eutectic cementite. The investigation was based on 200 samples each.

A camshaft with the module M = 0.53 cm, the necessary PG value of 0.99 was free of bubbles at 3.1% C, 2% Si, 0.15% Cr and 0.3% Cu, showed after remelting however, often inadmissible secondary graphite precipitations. By increasing the Cr content to 0.4%, there were still small amounts of secondary graphite which were tolerable for gasoline engines but already impermissible for diesel engines. This investigation is approx. Based on 50,000 camshafts.

A further increase in the Cr content led despite a simultaneous increase in the Si content in the as-cast state to increased carbide formation, which increases machinability has deteriorated inappropriately.

The same camshaft had 3.23 to 3.25% C, 1.7 to 1.8% Si, 0.15% Cr, 0.3% Cu, 0.05% Ti and 0.05% V none Secondary graphite, never shown in the wear test achieved results but had blisters. This investigation are approx. Based on 20,000 camshafts. By lowering the C content to 3.1 and increase in Si content to 2% Si corresponding to those identified in claims 1 and 3 The bubbles were no longer equations available. This investigation is approx. 5000 camshafts underlying.

Claims (5)

1. A process for the production of cast iron with lamellar graphite for at least partially remelt-hardened castings, characterized in that the cast iron (in mass%) 2.8 to 3.25% carbon
0.1 to 0.4% chromium
0.3 to 0.5% copper
at most 0.15% sulfur
[1.7 (% S) + 0.2] to [1.7 (% S) + 0.3]% manganese
a silicon content within a range of 1.8 to 2.5% in an amount according to the relationship% Si = 0.334 · +
+0.334 P · _G · -2.48 (% C) -0.01 · M · + 0.615 (% Cr)
-0.27 · (% Cu) + 5.6 ± 0.1 with the graphitization potential P _G = 0.657M ^-0.65 , and
M (module in cm), this is the quotient of the volume fraction of the heat-conducting surface of the casting,
is alloyed. 2. The method according to claim 1, characterized in that of SiO₂ to (FeO + MnO) is less than or equal to 2 and the equilibrium temperature of the SiO₂ reduction by carbon to 1380 ° C or greater can be set. 3. The method according to claim 1 or 2, characterized in that the Cast iron additionally 0.03 to 0.06 mass% titanium and / or 0.03 to 0.08 mass% vanadium be alloyed. 4. The method according to any one of claims 1 to 3, characterized in that the Aluminum content (in mass%) to 0.004 to 0.0090% and Al₂O₃ set to at least 0.0005% will. 5. Cast iron camshaft for internal combustion engines remelt-hardened cam surfaces, manufactured according to the method according to one of claims 1 to 4.