EP1566454A1 - Process for producing nodular cast iron products and products obtained - Google Patents

Abstract

Production of a workpiece made from a bainitic spheroidal cast material comprises austenitizing at 950-1100[deg] C in a carbon-containing atmosphere, cooling to room temperature and bainitizing by heating to the tempering temperature of 120-600[deg] C. An independent claim is also included for a workpiece made from a bainitic spheroidal cast material.

Description

The invention relates to a method for producing a workpiece made of bainitic nodular cast iron, starting from a nodular cast iron workpiece with spheroidal graphite by austenitizing and subsequent bainitizing. Bainitic nodular cast iron has opposite ordinary nodular cast iron and forged steel thanks to its special structure at the same Elongation at break a higher tensile strength and above all a higher fatigue strength under dynamic load; only alloyed steel is enough Properties. Further advantages are lower specific weight, good damping properties. Its thermal expansion is bigger, similar to that of aluminum and its alloys, which when combined this two materials is an advantage. Therefore this material is suitable for shafts, gears, cams, control cams, gearboxes and chassis parts.

Ductile iron is from the standards DIN EN 1564 (1997), ASTM A 897-90 (1997) ("Standard Specification for Austempered Ductile Cast Iron"), SAE J 24770 (2002) and ISO 17804 (draft) and known in these specified. He usually, but not quite, as a Bainitic Ductile iron or better called "Austempered Ductile Cast Iron". at its production is based on alloyed ductile iron, the Austenitizing takes place in a protective gas atmosphere at temperatures between 800 and 900 degrees Celsius and lasts at least two hours, at Parts with large wall thicknesses considerably longer. To the following Bainitize the workpiece from the austenitizing in a salt or metal bath to a bainitization temperature of 235 cooled to 425 ° Celsius for 1 to 4 hours at this temperature held and then quenched. This is initially disadvantageous that the alloying elements molybdenum, copper and nickel in the required Even before the heat treatment, make the workpiece heavy workable and are expensive. Much more expensive but come the long Dwell times in special elaborate devices (a special oven with protective gas atmosphere for austenitizing and a temperature-controlled Salt bath for bainitizing). In the structure, however, remains a tangible Proportion of needle-like ferrite responsible for the mechanical properties unfavorable.

Thus, it is an object of the invention to counter these disadvantages and bainitic To cheapen ductile iron and to improve its properties. For this purpose, a method is specified and accompanying to the other Improvement of the characteristics of the workpiece Composition of the workpiece proposed.

• Austenitisieren bei einer Temperatur von 950 bis 1100° Celsius in einer C-haltigen Atmosphäre,
• dann Abkühlen auf Raumtemperatur, und schließlich
• Bainitisieren durch Erwärmen auf eine Anlasstemperatur von 120 bis 600° Celsius.

The method according to the invention consists of the following three steps:

• Austenitizing at a temperature of 950 to 1100 ° Celsius in a C-containing atmosphere,
• then cool to room temperature, and finally
• Bainitize by heating to a tempering temperature of 120 to 600 ° Celsius.

Due to the higher Austenitisierungstemperatur goes in this temperature range significantly more carbon from the spheroidal graphite in the forming Austenite in solution, and in a much shorter time. The short required Holding time at this elevated temperature prevents the formation embrittled coarse grain. Cool to room temperature is not a problem and requires no special precautions. So A particularly fine austenite structure is created. The expenditure on equipment is thus cheaper and the occupancy time of the apparatuses shorter. When bainitizing are due to the wide temperature range and the relatively short Residence time the mechanical workpiece properties in a wide Process-reliably adjustable. Overall, in this process between austenitizing and bainitizing to room temperature cooled. In other words, that means the bainite area (also called "bainitnase") in the time-temperature conversion graph (in short: ZTU diagram) not from above or from the side (as with the State of the art) but from below, so with increasing temperature is reached. This leaves no ferrite and only as much martensite in the structure back, as are formed from the carbon formed in austenite can. The latter, because the martensite line because of the high Austenitisierungstemperatur in the ZTU chart and because of the higher carbon content the Austenite lies deeper. That brings a significant improvement of the structure and overall the mechanical-technological Characteristics with it.

The following process parameters have proved favorable: Austenitizing according to a) a residence time of 0.2 to 6 hours in one Atmosphere of 0.35 to 1.2 weight percent carbon (claim 2), where the lower value stands for very small and the higher value for very large and thick workpieces, such as a turbine rotor. The carbon atmosphere Prevents decarburization of the edge zones, so that the Workpiece is homogeneous in its entire depth.

The speed at which cooling to room temperature according to b), should be chosen so that the cooling curve in the ZTU graph the "Perlitnase" just not touched (claim 3). That means, the Cooling can be done at a reduced cooling rate, which (depending on the workpiece thickness) with a gas stream (nitrogen, helium and their mixtures, or even air) can be achieved again with little equipment expense. This is despite the higher austenitizing temperature the risk of distortion much smaller. For lower requirements to the dimensional stability of the workpiece is also a cooling in the Oil bath possible. For workpieces that require maximum wear resistance arrives, or in which the Perlitnase projects far, the Cooling rate chosen so that the cooling curve also the "bainitnase" just not touched (claim 4)

The bainitizing according to c) then takes place "from below", wherein during 0.5 up to 4 hours at the temperature of 120 to 600 ° Celsius the strength properties be set (claim 5). The duration is again depending on the size of the workpiece, for smaller shafts and gears it is rarely over an hour. In the wide temperature range are the mechanical workpiece properties by choice of temperature process-reliably precisely adjustable, and for maximum wear resistance between 120 and 250 ° Celsius (claim 6) and for high toughness and especially high fatigue strength between 250 and 600 ° Celsius (Claim 7).

The low risk of distortion also has a special advantage: the workpiece which is assumed to be made of ductile iron and can already be processed to its final form (claim 8), which in the ordinary spheroidal graphite iron, ie the unalloyed starting material in particular easy and fast goes (claim 9).

The process of the invention may be of ordinary ductile iron (For example, according to DIN EN 1563) go out (claim 9), which is no special Contains additional elements. In addition, in development of the invention the properties of the workpiece through a modified composition be further improved. Such an improved starting material Contains 2.8 to 3.6 weight percent carbon in addition to the iron and 2.7 to 4.0 weight percent silicon, balance unavoidable Impurities and impurities present in ordinary ductile iron (Claim 10). The basic values of the areas of Carbon and silicon crosswise to each other: To the higher Carbon content belongs to the lower silicon content and vice versa, so corresponding to the carbon equivalent, a composition slightly above or below the eutectic setting.

The increased carbon content is due to the higher austenitizing temperature possible. The increased silicon content prevents carbide formation while bainitizing. An already existing in ordinary ductile iron Addition is about magnesium, from experience 0.03 to 0.06 weight percent is sufficient to form the carbon in spherical form bring to. Particularly good values were achieved when the workpiece 3.1 to 3.5 weight percent carbon and 3.0 to 3.5 weight percent Silicon contains (claim 11).

Contains the workpiece further 0.03 to 0.06 weight percent calcium and / or cerium (claim 12), the spheres are made of nodular graphite and the distances between them are especially small. That makes a special fine and uniform texture. In addition to the already significant improved mechanical properties even the static Yield point can be particularly high, the workpiece finally still 0.2 to 0.6 weight percent nickel contain (claim 13), from Cost reasons preferably 0.2 to 0.35.

Fig. 1:

Ein erfindungsgemäßes Werkstück als Beispiel,

Fig. 2:

das erfindungsgemäße Verfahren im ZTU - Schaubild,

Fig. 3:

das erfindungsgemäß erzielte Gefüge im Schliffbild,

Fig. 4:

einen Vergleich der Festigkeitseigenschaften des neuen Werkstoffes mit dem Stand der Technik,

Fig. 5:

ein Schaubild der beim Bainitisieren einstellbaren Härte.

In the following the invention will be described and explained with reference to figures. They show:

Fig. 1:

An inventive workpiece as an example,

Fig. 2:

the method according to the invention in the ZTU diagram,

3:

the microstructure achieved according to the invention in the micrograph,

4:

a comparison of the strength properties of the new material with the prior art,

Fig. 5:

a graph of adjustable during bainitization hardness.

As an example, the production of a workpiece according to the invention described. It can be a crankshaft, camshaft or balancer shaft an internal combustion engine to gears, cams, Parts of a machine tool, or highly stressed housing or Chassis parts of a motor vehicle act. So made parts can forged, case-hardened, nitrided, otherwise hardened or tempered Parts or such of a material according to the prior art replace.

Fig. 1 shows, for example, a balancing shaft with gear. It consists of a shaft 1, a one-piece with her gear and from the bearing surfaces 3.4. The workpiece in its original shape is brought into shape by casting as ductile iron. Then, the gear 2 and the bearing surfaces are processed 3,4, which is very easy and fast because of the good Zerspannbarkeit in this state. If the known increase in volume during bainitizing is taken into account, the cast workpiece can already have the final shape, including a coarser toothing. At best, then after the heat treatment, the bainitizing, no mechanical processing at all necessary.

Ductile iron is either a heat-treated ordinary invention Ductile iron according to DIN EN 1563 or one according to the invention modified and treated ductile iron. Such arises through Adding further elements (Ca, Ce, Ni) or by extending the Percentages of carbon and silicon. The following are typical analyzes concrete examples of both:

Example 1: Ordinary ductile iron

C = 3.74% Ni = 0.0% Si = 2.83% Mg = 0.056% Mn = 0.2%

Rest Fe and inevitable admixtures.

Example 2: modified ductile iron

C = 3.33% Mg = 0.031% Si = 3.77% Ca = 0.04% Mn = 0.11% Ce = 0.03% Ni = 0.29%

Rest Fe and inevitable admixtures.

Workpieces with these two analyzes were then the inventive subjected to two-stage thermal treatment, like conventional case hardening in conventional chamber furnaces or blast furnaces can be done. Special metal or salt baths are not required.

In the ZTU diagram of FIG. 2 , the time is plotted on the ordinate and the temperature on the abscissa. The fields assigned to the various microstructures are denoted by A (= austenite), B (= bainite), F (= ferrite), M (= martensite), P (= perlite) and M _s (= martensite start line). The bainitization according to the prior art takes place along the dashed curves a, b and c.

According to the invention, the workpiece is made of ordinary or improved Ductile iron for austenitizing initially heated to 1000 ° C, to prevent superficial decarburization in carbon enriched The atmosphere. Due to the increased temperature, the increased Carbon content mostly in solution. Their effect is through a numerical example to show that (according to Darwish and Elliott) the carbon content Compare Cy in the austenite so formed for the same workpiece: At 900 ° C and a residence time of 330 minutes Cy = 1.27%, At 1000 ° Celsius and a residence time of 60 minutes Cy = 1.16%, At 1050 ° Celsius and a residence time of 60 minutes Cy = 1.32%, Accordingly, for example, the workpiece is 0.2 to 2 hours on a Temperature of 1050 ° C kept.

As a result, it is accelerated, but to avoid distortion of the workpiece is not too fast, cooled to room temperature, according to the curve 7, to the point 8. You can see that they are far from the "perlite nose", which is essential to avoid but passes very close to the "BainitNase" B, to a line M _s-new which is lower because of the higher carbon content at the higher austenitizing temperature. Because of the deep line M _s-new , the martensite content and therefore also the embrittlement is very low. The content of retained austenite, which forms a ductile matrix, also contributes to this. Cooling occurs depending on the circumstances and size of the workpiece by oil or high pressure gas.

Finally, the bainitization follows by tempering at 120 to 600 ° Celsius and for 0.5 to 4 hours. The temperature depends on the required mechanical and technological properties (hardness, toughness, Fatigue strength, tensile strength, etc), the time depends mainly from the mass to surface ratio of the workpiece. In FIG. 2 shows two, for example, courses. The trapezoidal Curve 9 shows the temperature curve when bainitizing to 300 ° C, curve 10 when bainitizing to 530 ° C.

The mechanical properties of a treated according to the curve 9 at 300 ° C. Workpiece are high wear resistance due to hard surface (e.g., 56 HRC) at moderate elongation at break. One after the curve 10 At 530 ° C treated workpiece shows about twice the fatigue strength of a spheroidal cast iron according to the prior art (DIN EN 1563), a high Core strength, because of the smaller modulus of elasticity, a very high noise damping capacity and because of the very small carbon spheres (Sphärolithen) hardly notch effect and therefore high fracture toughness, lower weight (density 7.1 g / cm3, compared to 7.85 of steel), good castability and a coefficient of thermal expansion that the of aluminum comes very close. Because of the thermal expansion coefficient due to the noise damping capacity ideal for Gears and shafts running in a light metal housing.

Fig. 5 finally shows the relationship between the tempering temperature = Bainitisierunstemperatur and the surface hardness set (HB). As expected, the hardness drops with the tempering temperature.

The resulting structure is shown greatly enlarged in Fig. 3 . It is a very fine mixture (gray in the picture) of austenite and bainite, with only a few martensite needles (white in the picture) and very small graphite beads (black in the picture).

For comparison, the properties of elongation at break and tensile strength of different materials are compared in Fig. 4 , the former on the ordinate and the second on the abscissa. Field A corresponds to the prior art bainitic nodular cast iron, B forged alloy steel, C forged mild steel, D ordinary nodular cast iron, and E a nodular cast iron bainitized according to the present invention.

Claims (13)

Process for the production of a workpiece from bainitic ductile nodular cast iron starting from a work piece made of nodular cast iron by austenitizing and subsequent bainitizing, characterized in that the following steps are carried out: a) Austenitizing at a temperature of 950 to 1100 ° Celsius in a C-containing atmosphere, b) then cool to room temperature, and finally c) Bainitizing by heating to a tempering temperature of 120 to 600 ° Celsius. A method according to claim 1, characterized in that for austenitizing according to a) a residence time of 0.2 to 6 hours in an atmosphere of 0.35 to 1.2 weight percent carbon is maintained. A method according to claim 1, characterized in that the cooling to room temperature according to b) is carried out with a speed selected such that the cooling curve (7) in the time-temperature conversion diagram (Fig.2) the "Perlitnase" (P) just not touched. A method according to claim 1, characterized in that the cooling to room temperature according to b) takes place with a speed selected such that the cooling curve (7) in the time-temperature conversion diagram (Figure 2) the "bainitic nose" (B) just not touched. A method according to claim 1, characterized in that the strength properties are adjusted by bainitizing according to c) for 0.5 to 4 hours at a temperature of 120 to 600 ° Celsius. A method according to claim 5, characterized in that by bainitizing according to c) takes place at a temperature of 120 to 250 ° Celsius. A method according to claim 5, characterized in that by bainitizing according to c) takes place at a temperature of 250 to 600 ° Celsius. A method according to claim 1, characterized in that the workpiece is assumed that is already processed. A method according to claim 1, characterized in that the workpiece, which is assumed, consists of ordinary nodular cast iron. Spheroidal cast iron workpiece, characterized in that it contains, in addition to the iron, 2.8 to 3.6% by weight of carbon and 2.7 to 4.0% by weight of silicon, the remainder unavoidable impurities and admixtures present in ordinary ductile iron. Workpiece according to claim 10, characterized in that it contains 3.1 to 3.5 weight percent carbon and 3.0 to 3.5 weight percent silicon. Workpiece according to claim 10, characterized in that it further contains 0.03 to 0.06 weight percent calcium and / or cerium. Workpiece according to claim 10, characterized in that it further contains 0.2 to 0.6 weight percent nickel.

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