Scale Reduction in Hot Forging by Dense Coating

Transcription

1 Scale Reduction in Hot Forging by Dense Coating Dr. Helmut W. Seidel 1 Abstract Scale formation leads to material defects, tool damages (wear), material losses in hot forming operations of steel. Decrease or preventing the scale formation leads better quality of the forgings, improved tool life, less aftertreatment and reduced rejects quantity. In order to achieve these goals, with LUBRODAL SR 300 a new, waterbased, environmental friendly coating, was developed which is applied before the heating up simply on the billets or bars. Results from practice show advantages and above all economy in the comparison to the state of the technique (inert gas, glass coatings and others). The practical results are occupied by extensive laboratory tests, which are presented briefly. Relative forging operation and used steel grades on the basis of examples the possibilities in addition, the borders are discussed. 1 Introduction & Basics Scaling is a great annoyance when hot forging steel and related alloys (Figure 1). Scale starts being produced when the material is heated to C, increasing during the forge operation up to the cool-down phase of the parts [1]. Oxidation of the steel components causes not only significant material losses, but also negatively influences the reshaping process. Defects on the surface of the finished parts require subsequent reworking and/or increase the rejection rate. The abrasive scaling leads to increased tool wear and reduces the lifetime of the dies. The expensive reshaping tools are a meaningful cost factor for the total production process. Frequent tool replacement, reworking, defects in quality and rejection reduce productivity, and with it the profitability of the process Figure 1: Scale 1 FUCHS LUBRITECH GMBH Werner-Heisenberg-Straße Kaiserslautern/Germany Phone: +49 (0) Fax: +49 (0) helmut.seidel@fuchs-lubritech.de

2 In order to hinder the development of scaling, oxygen must also be hindered from attacking the surfaces of the hot materials [2]. The current state of the technology presents two basic approaches to solve this problem [3,4,5]: I. Protective gas atmosphere (nitrogen, argon) II. Coatings on the material surface (preferably glass coatings) The practiced procedure with inert gasses (I.) is primarily limited to heating and cooling. Attempting to perform complete handling and the actual reformation in protective gasses is only possible with great technical effort. Especially for economic reasons, this method is impractical for mass production. The second variant (II.) has been successfully used for many years to forge special alloys [6]. Glass dispersions, however, also have their price and require additional application and use technologies. Here as well, economic factors speak against widespread use. For this reason, it has been and remains an understandable challenge to the steel industry to reduce the formation of scaling when reforming steel through meaningfully cheaper and simpler methods. As a producer of specialty and forming lubricants, FUCHS LUBRITECH set upon the concrete goal of developing appropriate surfaces for this purpose. 2 Development Work 2.1 Requirement Profile With the mentioned basic information from practice, the following requirement profile resulted to develop a new, powerful coating for forged steel: Simple and economical to use Water-based, ph-neutral, environmentally friendly High thermal stability, good adhesive qualities No development of harmful gasses or flammable products Low friction Compatible with die lubrication No dangerous influence on induction and refractory Harmless to humans and the environment General availability In order to fulfill the goals named above, a number of comprehensive initial tests were necessary. 2.2 Thermogravimetric Measurements For the selection of ingredients, from the very start only inorganic compounds and mineral materials came into question. Legislation on chemicals, availability and - 119

3 physical properties combined with considerations on price to narrow the original circle of candidates. Further, a selection needed to be made from a multitude of compounds. Thermogravimetric measurements presented themselves for this purpose. Figure 2: Thermogravimetry of inorganic components We decided to evaluate the mass losses (mainly chemically bound water) up to 1000 C as the decisive criterion. Materials that appeared appropriate were then intensively tested for scaling reductions in a laboratory and tested in parallel in industrial experiments. These tests took place in 20 forging shops with varied reshaping processes while pressing, forge rolling, and hammering. Screening and targeted evaluation led to a commercial product with the following properties. 2.3 Product Properties Lubrodal Sr Figure 3: Physical and technical data 4

4 2.4 Press-In Test The influence of the coating on the following forging process was examined on a company-internal testing station. On it, a plated steel bolt was pressed through a steel sleeve heated to 900 C. The diameter of the bolt is 0.5 mm larger than that of the sleeve. The measured press-in force was, at 75 kn, on the same order of magnitude of graphite-free die lubricants. The process took place nearly inaudibly and in the absence of any stick-slip effect, which is undesirable for reshaping. These results afterwards confirmed the positive effects of the coating on the forging process. Figure 4: Scheme Press-In Test Figure 5: Press-In graph LUBRODAL SR

5 3 Results In Practise 3.1 Application Methods LUBRODAL SR 300 is to be used in pure form and can be applied using sprays, immersion, showering, and brushing on the cold material (billets, bars). The surfaces of the material to be coated must be free of greases and oils. Sandblasted parts are very advantageous, due to the more raw surface structure, to pick up the separating layer and for better adhesion. Automatic spraying is the most costeffective and evenly distributed application method. The average amount of LUBRODAL SR 300 through spray application can be calculated as requiring a surface density of g of product per square meter Figure 6: Application by spraying The plated billets should, if possible, be applied on-line to heating and forging without interim storage. Since the coating is weakly hygroscopic, longer periods of storage in moist atmospheres can lead to slight emergence of corrosion on the precoated parts through water absorption. 3.2 Mechanical Press Forging Multi-stage press (EUMUCO MP 3.150), round parts (3-5 kg); parts / min, 3 5 stations; 1200 C (induction heating), LUBRODAL SR 300 pure by spraying (Figure 6) Results: up to 15% less scale, better lubrication, higher die life, better quality, less rework and rejects 3.3 High Speed Forging (Horizontal Press) Hatebur AMP 20; AMP 30 smaller parts (< 1 kg); part / min induction heating LUBRODAL SR 300 pure by drop-flooding 6

6 Figure 7: Scheme drop-flooding Results: up to 10% less scale, better lubrication, no negative influence on re-circulating fluid, higher productivity by less tool changes 3.4 Hydraulic Press to dishing (coning) press, railway rods ( kg), LUBRODAL SR 300 by brushing before second heating (gas oven) Results: less marks, better surface quality, less reworking 4 Further Investigations 4.1 Steel Grades First results from the lab and from practice showed from early on that the SR 300 coating is not appropriate for high quality steel grades to minimize scaling. Indeed, it resulted in increased scale growth. Quality steels and special alloys require a different concept for the coating. For low-carbon steel grades like those primarily used for conventional warm forged parts, on the other hand, up to 25% less scaling is produced on average in comparison with untreated reference samples based on experimental tests. Additional extensive thermogravimetric measurements and individual examinations were necessary on this point. 4.2 Investigation On Diffusion Effects A question which is often asked from practice was also how much the chemical components diffused into the steel matrix. To answer it, combined REM / EDX measurements were launched (Figure 8). The evaluation of the diagrams showed conclusively that the components of the coating were found only in the scale and on the surface

7 124 - Figure 8: REM / EDX diagram Two zones (lines) for scale and surface were detected, with a thickness of 10 µm each. Scale is easily removed. Slight decarburisation was found. 5 Summary and Outlook LUBRODAL SR 300 provides a coating for use in the hot forming of conventional forged steels that minimizes scaling. This ready-to-use material is water-based, environmentally friendly and simple to use. The components of the coating do not diffuse into the steel matrix. They are located exclusively in the layers of waste scale. Lasting and reproducible results were sought by performing an automatic spray application on fast-moving forge presses with inductive heating. Statistically and cumulatively in quantity, reduced post-processing, better part quality, and longer die life are the result. Productivity and economy of the process increases. LUBRODAL SR 300 supports die lubrication and does not develop residue in the dies. After many years of use in practice, no negative influence on induction coils, oven atmosphere or refractory have been found. The range of application covers further areas of steel hot forming. Thus, recently, hot rolling and open die forging methods have been developed. For higher quality steel alloys (high carbon steel and special metals), another concept of coating is required. First results with a new formula (LUBRODAL SR 900) show promising areas of application. It results in less scaling. Its mechanical removal is easier. The skin decarburization is significantly reduced. It inhibits billet adherence while passing through the oven. 8