INFLUENCE OF PRELIMINARY AND FINAL HEAT TREATMENT ON STRUCTURE AND HARDNESS OF THE TSEMENTOVANNOY STEEL 20Õ

Gorbatenko V.P., Konareva S.V. (the FM department)

The rise of firmness of details of press instrument for making of wares from heat-resistant materials is an enough important task. For the flags of pressform, which work in the conditions of abrasive wear, for an economy in place of the staley type H12 in a number of cases apply tsementuemie to steel with consolidating by their himiko-termicheskoy treatment In accordance with the operating requirements for providing of necessary wearproof pressform hardness of surface of flags must be no less than 59 HRC. At the same time in practice, the cases of receipt of the lowered hardness of working surfaces of flags after HTO are marked. The possible reasons of this can be related both to violations of technology of consolidating treatment, and with influencing of the initial structural state and terms of preceding treatments.

With the purpose of establishment of reasons of insufficient firmness of working elements of press instrument and subsequent adjustment of existent parameters of consolidating treatment a structure and hardness of the flags of pressform, made from sheet steel 20Õ after treatment in the conditions of operating production on the following modes, was explored:

Mode 1. Cementation in hard karbyurizatore at a temperature 920-  950^oC with self-control 10-12 hours, cooling on air.

Mode 2. Tempering from the cementation heating after podstugivaniya to 870^oC.

Mode 3. Tempering from the cementation heating after podstugivaniya to 800^oC.

Mode 4. Treatment on the mode 2 + tempering from the separate heating to 800^oC.

The study of structure of standards after treatment on the mode 1 allowed to set that the depth of tsementovannogo layer on the whole responds to request (h ³ 1,5 mm), however there is a different depth of layer from different sides of flags. Thus, on one side of flags a depth could make 3.3,5 mm, and on other - 1,5.1,6 mm. The reason of heterogeneous satiation can be the inefficient piling of flags in a box for cementation. The given measuring of hardness, presented on a picture 1, testify that hardness after cementation made 35-45 HRC, and after treatment on the mode 2 was found at a level ~58 HRC, consolidating on the mode 3 provides the receipt of hardness 60 - 62 HRC, and on the mode 4 - 62 - 63 HRC.

         The results of rentgenostrukturnih researches allow to assume that the decline of hardness of steel hard-tempered from the cementation heating can be related to the promoted maintenance of remaining austenita in a superficial layer, which made ~41%. At the study of structure of tsementovannogo layer after tempering from the cementation heating the presence of coarse-grained structure with middle measuring corns was exposed ~ 71,4±15 mkm. Conducting of tempering from the separate heating resulted in the receipt of more than dispersion structure of austenita with middle one measuring corns 10-17 mkm. The analysis of frequency distribution curves of corns of austenita on the sizes presented on a picture 2 shows that the more than shallow and more than even corn of austenita in a tsementovannom layer is formed after the double tempering.

In connection with that the flags of pressform are made from a sheet, including got and the controlled rolling, in the initial state a metal is characterized by the presence of developed structural poloschatosti, which was not fully removed even after the protracted self-control during cementation. Forming of raznozernistoy structure of austenita in the process of cementation resulted in the receipt of raznozernistoy ferrito-perlitnoy structure of metal of serdtsevini and transitional area after cementation and slow cooling. In a structure the large on sizes areas of perlita, located in the fields of dispersion ferrito-perlitnoy structure, were visible. It can serve as a certificate of presence of very large corns of austenita in the structure of steel before diffusive transformation.

In order to additionally affect a structure in the initial state, before  cementation  conducted  preliminary  thermal

treatments - annealing or tempering. The complete cycle of consolidating heat treatment in this case set following modes:

Mode 5. Annealing at 870^oC before cementation, cementation with the subsequent cooling on air, tempering from the separate heating from 800^oC.

Mode 6. Tempering from 870^oC before cementation, cementation with the subsequent cooling on air, tempering from the separate heating from 800^oC.

Research of microstructure of standards after such treatments allowed to set that after the mode 5 the raznozernistaya structure  of krupnoigolchatogo martensita is formed, and after treatment on the mode 6 martensit had the more than fine-grained structure, that, obviously and was instrumental in the receipt of more than high hardness 61 - 63 HRC, than after the mode 5 - 59 - 60 HRC.

Thus, the positive influencing of tempering is experimentally confirmed from the separate heating to the structure and properties of tsementovannoy steel 20Õ after cementation in hard karbyurizatore.