DE3442130A1 - Process for surface-hardening into the bainite zone of castings of ductile iron-carbon casting materials - Google Patents

Abstract

Castings with a bainitic-austenitic structure have a higher strength and fatigue strength and better wear resistance than the ferritic to perlitic grades. In order to avoid the poorer machining properties and the delay of a bainite hardening throughout, surface-hardening of only the particularly stressed regions into the bainite stage has already been carried out, namely by inductive heating and quenching in hot oil or in salt baths. The expensive hot baths are avoided according to the invention by preheating the castings as a whole to a temperature approximately corresponding to the intended transformation temperature and rapidly heating the surface zones or casting regions to be hardened to a temperature from 880 DEG to 950 DEG C. From these, the heat rapidly flows off into the only moderately preheated interior of the casting, so that the austenitised regions have to be cooled only slightly by moving air or spray mists down to the vicinity of the intended transformation temperature. Immediately afterwards, they are transferred into a furnace preheated to the transformation temperature. Because of the short delay, machining can be carried out before the surface-hardening.

Description

Process for surface hardening in the bainite stage of castings from ductile iron-carbon cast materials The manufacture of castings with complete bainitic-austenitic structure made of ductile iron-carbon cast materials, like cast iron with spheroidal graphite and malleable cast iron, is becoming increasingly important. Higher Strength, elongation, fracture toughness, fatigue strength and better wear resistance this group of materials distinguishes itself from the ferritic to pearlitic types the end.

In technical applications, however, the machining properties are inferior - especially when drilling - and the distortion with a continuous bainite coating disadvantageous.

These disadvantages can be avoided if not the entire casting is continuously bainite-tempered, but only the particularly stressed areas be subjected to a surface hardening in the bainite stage. Has been described such a way of working by K. Hornung (1), who used a partial bainite coating after induction heating and quenching in HeiBB1 (235/250 ° C). The use of salt baths is also an option for this purpose.

However, the use of hot baths is a technical effort which can be avoided by the method according to the invention.

This consists in the fact that the piece is continuously heated to about the temperature z. B. 250 to 420 ° C is preheated, at which the later largely isothermal Conversion should be carried out. The parts to be hardened in the bainite stage of the GuBstOcks are immediately thereafter with a heat source, z. B. a flame, an electric arc or a plasma jet or in an induction hardening device heated to surface temperatures of 880 to 950 0C and immediately afterwards with moving air or spray to approximately the intended transformation temperature cooled down. This can be done with several parts to be hardened at the same time or - z. B. in the case of gears - done sequentially. The preheating depends on the Geometry of the cast and according to the heating volume of the cast parts to be hardened. In the case of thin-walled castings and a low hardening depth, the preheating temperature is to choose something higher than the transition temperature. For compact castings that The preheating temperature is to be lower bainite tempered in larger areas lower than the transition temperature. Target of these temperature deviations the preheating is off the austenitized area the heat dissipation in the base material on the one hand and in the moving air or the spray mist on the other to steer in such a way that when the bainite begins to form, the intended isothermal Conversion temperature has largely set.

Immediately after reaching the transformation temperature into the austenitized In the surface areas, the cooling is stopped and the casting is placed in an oven transferred, which has already been adjusted to the intended transformation temperature beforehand became. The casting remains for 10 to 120 minutes in this furnace in the case of cast iron with spheroidal graphite and 10 to 30 minutes for malleable cast iron. The time required is essentially based on this according to the alloy used. Unalloyed materials are largely in 10 to 30 minutes converted, while materials containing Cu, Ni and Mo in the upper concentration ranges are alloyed, require conversion times between 60 and 120 minutes. Here is the Constant influence of alloying elements.

The advantage of the method according to the invention is to be seen in the fact that, due to the lower distortion, machining after the surface layer treatment is normally not required. The introduction of holes, for. for oil cow channels of crankshafts, does not encounter any difficulties, since in the critical areas of coarser drilling depths the original ferritic to pearlitic structures are present, which do not cause any difficulties. The tempered edge layers consist of a bainitic-austenitic matrix which has the technological advantages described at the beginning and thus gives the casting the advantageous properties mentioned at the beginning in the critical areas. The following chemical compositions can be considered for the ductile materials cast iron with spheroidal graphite and malleable cast iron: C Si Mn S Cu Ni Mo %%%%%%% GGG min 3.2 1.8 0.1 - 0 0 0 max 3.8 30 0.6 0.015 0.8 1.0 0.5 Malleable cast min 2.5 1.2 0.5 0.12 0 0 - GTS mex max 2.8 1.5 0.8 0.19 0.6 0.5 - In the initial state, the bound C contents should be> 0.5 X in order to remedy the formation of an austenite with sufficiently high C contents.

First example: A car crankshaft had the following composition: C Si Mn P S Cu Ni Mo%%%%%%%% 3.73 2.08 0.34 0.030 0.010 0.34 0.66 0.33 Sie was preheated to 370 ° C in a circulating oven, in an induction hardener inserted and a main bearing area with 60 mm W at approx. 900 0C approx. 2.5 to 3.0 mm heated deeply and austenitized for 40 seconds.

Immediately afterwards, the temperature was raised with a ring-shaped air shower Controlled by an IR temperature measuring device, cooled to approx. 480 ° C and the piece placed in an oven preheated to 380.degree. After a 40 min hold time, the crankshaft taken and examined. The hardness and structure development was as follows: Distance in mm 0.2 0.5 1.0 2.0 3.0 4.0 HV 10 268 280 2.5 275 240 240 Structure 8-A 3-A B-A S-A P-F P-F 8 bainite, A austenite, P pearlite, F ferrite Second example: the tubular End of an unalloyed (3-base made of GGG-70 with the dimensions 116 # # 8 approx. 60 mm long should be tempered with a bainitic edge layer on the front surface and on the outer circumference will. The composition was: C Si Mn P s Mg%%%%%% 3.6 2.6 0.3 0.050 0.00B 0.034 The casting was preheated to 450 C and the tubular end inductive heated to approx. 920 ° C. Immediately thereafter, the austenitized area was through a water spray cooled to approx. 400 0C and the casting in one to 350 C preheated air circulation oven for 15 min held. Hardness and structure achieved the following values starting from the face: Distance in mm 0.2 0.5 1.0 2.0 3.0 4.0 Hardness HV 10 320 330 325 280 275 280 Structure B-A B-A B-A P P P Hat a larger casting has sufficient heat capacity, so a local bainitiache Surface hardening can even be carried out without an oven: Example 3 A 5 t heavier Machine stand made of GGG-50 had the following chemical composition: C Si Mn P S Ni Cu Mo Mg%%%%%%%%% 3.55 2.26 0.28 0.021 0.006 0.8 0.45 0.38 0.0 '.

An area subject to wear and tear, measuring 120 x 200 mm in one Wall thickness range of 60 mm should be treated with a bainitic surface layer. Of the The area around this area was preheated to approximately 350 OC with burners. The process was followed with an infrared meter. Then the area to be paid for became Austenitized to 880 to 930 C and immediately afterwards with spray to approx. 400 ° C cooled. Then the thermally stressed area was treated with an induction device, as it is used for the annealing of construction site welds, over about 80 min kept at a temperature between 340 and 380 0C. A hardness test and a Metallographic examination of the casting showed a Brinell hardness of 280 to 300 and a matrix consisting essentially of upper bainite.

When choosing the material composition, it is important that during During the cooling phase in the austenitized area, the material is set so that it is slow to transform (application of ZTU diagrams for continuous cooling) that no pearlite formation can take place.

Claims (9)

Claims: 1. Process for surface hardening in the bainite stage of castings made of ductile iron-carbon cast materials, characterized in that that the castings are preheated as a whole to a temperature that is about The transformation temperature provided corresponds to the parts of the castings to be hardened The parts to be hardened are quickly heated to a temperature of 880 - 950 "C then by applying a coolant to close to the intended transition temperature cooled and the castings then in a preheated to the transformation temperature Oven to be transferred. 2. The method according to claim 1, characterized in that the applied Coolant is moving air. 3. The method according to claim 1 or 2, characterized in that the applied coolant is a spray mist. 4. The method according to any one of the above claims, characterized in, that the castings made of cast iron with spheroidal graphite of the following chemical composition exist: C 3.2 to 3.8% C bonded # 0.5% Si 1.8 to 3.0 X Mn 0.1 to 0.6% Cu 0 to 0.8 X Ni 0 to 1.0 X Mo 0 to 0.5 X S i 0.015 X and others usual accompanying elements. 5. The method according to claim 4, characterized in that the castings made of spheroidal graphite cast iron in the preheated to the transformation temperature Oven can be held for 10 to 120 minutes. 6. The method according to any one of claims 1 to 3, characterized in that that the castings are made of black malleable cast iron with the following chemical composition consist of: C 2.5 to 2.8% C 3 0.5 Si 1.2 to 1.5% Mn 0.5 to 0.8% Cu 0 to 0.6 X Ni 0 to 0.5% Mo S 0.12 to 0.18 X and other common accompanying elements. 7. The method according to claim 6, characterized in that the castings held in the furnace preheated to the transition temperature for 10 to 30 minutes. 8. The method according to any one of claims 1, 2, 3, 4, 5 for surface hardening with larger and thick-walled castings made of cast iron with spheroidal graphite, thereby characterized in that the preheating is also carried out outside a furnace with burners and after the austenitization processes, accelerated cooling to approx Conversion temperature, the temperature constancy during the conversion time Maintain insulation of the casting and / or local heating will. 9. The method according to claim 8, characterized by nickel content up to 2.5%.