V8C7, (Fe3W3)C-(Fe4W2)C carbides in HS10-2-5-8 steel - Quenched from 1200 C/5 min

Table 1: Tested steel grades and their chemical composition.

Table 2: Amount of undissolved carbides and carbide types after heat treatment.

Carbide name: V8C7, (Fe3W3)C-(Fe4W2)C
Record No.: 716
Carbide formula: V8C7, (Fe3W3)C-(Fe4W2)C
Carbide type: M8C7, M6C
Carbide composition in weight %: No data
Image type: No data
Steel name: Bohler S390
Mat.No. (Wr.Nr.) designation: No data
DIN designation: HS10-2-5-8
AISI/SAE/ASTM designation: ~T15
Other designation: No data
Steel group: PM high speed steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: The samples for the electrochemical carbide extraction had a size of 20 mm diameter and 15 mm length. The samples were austenitized in the range from 900 C to 1250 C. According to the results of the equilibrium tests, the immersion times were 5 min for temperatures above 1150 C and 20 min for lower temperatures. Condition: Quenched from 1200 C/5 min.
Note: Thewear resistance of coldwork steels essentially depends on the amount and on the types of undissolved carbides present in the hardened condition. The aim of this paper is to show the differences of carbide content in tool steels for cold work used today. Only the content of undissolved carbides after heat treatment is important for wear resistance. Therefore the changes of carbide content by heat treatment have been determined. The rate of carbide solution at austenitizing temperature and the time to reach equilibrium structure were also tested.

The extraction of undissolved carbides after austenitizing was carried out electrochemically. In an electrolyte of 5 % muriatic acid and 95 % methyl alcohol the matrix of the samples is dissoluted with an intensity of current of 5 mA/cm². The dissolution potential of the matrix is about 500 mV and much lower than that of the carbides with about 900 mV. Therefore it is possible to dissolve the matrix completely without attacking the carbides. The electrolytic dissolution of thematrixwas done galvanostatically, because under these conditions a quantitative extraction of carbides can be expected.
The extracted amount of carbides can include small parts of the carbon dissolved in the matrix in colloidal solution. Thereby the measured amount of extracted carbides is a little too high. The fault in weight content is however less than 2 % of the mass of the extracted carbides. Therefore the fault was not taken into account at the weight of undissolved carbides. Whereas the total amount of carbides wasmeasured on all tested samples, the nature of the extracted carbides was only tested in the annealed condition and after austenitizing at 1150 C or 1200 C. The carbide identification was made by X-ray structural analysis and by use of the ASTM card index.

Interesting and remarkable is the influence of the size of the primary carbides on the dissolution rate. At all tested temperatures the dissolution rate in structures with coarse carbides is clearly higher than with smaller carbides. The following explanation can be offered. To manufacture a structure with coarse carbides, the billets have to be heated at higher temperatures before rolling. Thereby in these billets more carbides are dissolved than in billets which are heated at lower temperature. The amount of dissolved alloying elements does not diffuse back during rolling and the following heat treatment. They remain in the matrix and precipitate in form of very small carbides in the matrix. Due to the higher amount of very small carbides distributed in the matrix, the matrix is here faster saturated with alloying elements at hardening temperatures. Therefore steels with coarse primary carbides are easier to harden than those with a fine carbide structure. But this is only true, if the coarse carbides have been formed by coagulation at high temperature. If coarse carbides result from other reasons, e.g. by means of solidification as in HSS type AISI T18, one can not necessarily expect a quick carbide dissolution at austenitizing.

In the annealed condition the tested steel grades have carbide amounts between 15 and 30 weight %. In all tested tool steels the amount of undissolved carbides clearly decreases when the austenitizing temperature increases. The decrease of the amount of undissolved carbides mostly drops linear with increasing hardening temperatures. As is to be seen, the content of carbides is reduced by approximately 35 to 45 % when the annealed state is heated to 1200 C. When the amount of carbides drops, normally the wear resistance should also decrease. But one also has to take into account the type of carbide, that means the hardness of carbides. Table 2 gives some information of the carbide types to be found in the annealed and in the at 1150 Cor 1200 C hardened condition. Table 2 also shows how the types of carbides and the weight percentage of the carbide types are changed by the heat treatment. The weight percentage of the stable carbide type MC increases whilst the content of the less stable carbide typesM23C6 orM7C3 is reduced, changed to more stable carbide types or are completely dissolved.
Links: No. 715 and No. 716.
Reference: Not shown in this demo version.

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