Case study of technological problems arising from the use of new material, DOMEX steel sheet, on the components of the mower discs on the example of the production company SaMASZ Sp. Z o. o

The paper describes the improvement of technological process of knife holder production used in disc mowers at SaMASZ Sp. With o.o. The improvement was aimed at eliminating cracks in the press of a 500t crank press press crush that occurred after the Boron 27 starting material was changed to Domex MC 500 high strength steel. The technology of regeneration of claws with identified operational cracks and a stand for fatigue tests was also proposed. Defectoscopy and strength testing methods have been developed to verify that the structure of the regenerated material is free from cracks or micro cracks in the material.


Introduction
The subject of the publication is the problem of cracking knife holders used in Perfect CUT disc mowers during pre-pressing. The problem was due to the substitution of the Boron 27 starting material on the DOMEX 500MC high strength sheet [1].
Changing the material without adjusting the stigma of the handler and cutting system on the sheet during laser cutting resulted in numerous defects in the form of cracks, micro cracks and corrugations [2].

The construction and technological characteristics of the knife holder
The knife holder is one of the parts of the rotating disc, the cutting unit of the Perfect CUT cutter [3].

Fig. 1. Perfect CUT disc strip
This feature is designed to prevent knives from blocking during operation [4]. Initially made of material Boron 27 now made of DOMEX 500MC steel. Total weight is 1.519 kg, # 5 mm sheet.

Technological process of knife holder
SaMASZ Disc Mower Technology Processes:  005 -Cutting Sheet Sheet -LASER  010 -Cuts the disk holder -cracks and micro-cracks occur in the process -PRESSES  015 -Punch the holes of the disk holder -PRESS  020 -Finished (2 pages) -PRESS  025 -Temper and temper Afterwards, the bracelets are subjected to a hardening process and the cracks become even more visible in the process. The heat treatment process aims to improve the mechanical properties of the material by causing significant changes in their internal structure. However, during this process, the accompanying physicochemical phenomena may affect the material so that, instead of improving, it may impair the mechanical properties of the metal materials [5]. In this case, they may cause internal stresses, which will affect the strength of the object at the point of focus and cause permanent deformation and cracks. The above disadvantages associated with the hardening process lead to cracks in the structure of the material from which they are made, which results in an increase in their costs during production. Cracks in the heat-treated parts due to internal stresses create the greatest problems in the manufacturing process of finished goods [6].

Analysis, identification of cracks occurring during prepressing, corrective actions
At the end of 2016, during the hardness testing of parts after hardening of the quality control department, he noticed cracks and micro cracks in the holders. At the beginning of the study, two parties were analyzed. Then the separated holders -304 pieces were divided into groups shown in Table 2. Cracks were hardened after quenching -allowed for production 3 Cracks above> 3 mm 146 Not allowed for production

Target improvements to the technology process of the holder
SaMASZ made two changes to prevent defects in parts: A. Construction change -the handle has been redesigned, a change in the blank of the holder has been applied by increasing the radius ( figure 6). This change was strategic, because after this change, the crooks do not show cracks.

Endurance tests, bending simulations of the holder during blade replacement, ultrasonic defectoscopy
For the technological research of the holders we used 8 samples made in SAMASZ company. Hardened central knife holders with cracks after pressing, were prepared for checking resistance to cyclic bending-real work simulation using a specially shaped key. View of the station and the way of simulation is shown in Figure 9.    Defected holders were regenerated by welding cracked places by MAG method. The next step was to grinding welds equally with the holder. To make sure if regenerated holders are ready to use in production, We made bending tests for holders exactly with the side of insert and remove of the holders knife.

Description of research results
Test conditions:  temperature = 23 º C  Frequency = 2 s  surface condition -surface cleaned, ground  hardness of the material (DOMEX 500HB)    Figure 10 we see that the stick exhibits discontinuity of material. From ultrasonic flaw detection, we read a distance value of 2.1 cm. The distance of the center of the head from its origin is 1.4 cm, so the microcracks are 0.7 cm from the beginning of the head [7].

Conclusions
The analysis of endurance tests and ultrasonic flaw detection allowed us to formulate the following conclusions:  change in the technological and organizational process used to produce the knife holder, eliminating defects in the manufacture of the work piece;  regenerated by welding (MAG method) knife holders, can be used for further production.