TECHNICAL ARTICLE:

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Introduction

The application of thin abrasion resistant layer or a metallic surface coating over a substrate provides increased wear resistance, avoids surface damage, or modifies the frictional behaviour. The selection of an appropriate surface treatment process for mandrels requires a systems engineering approach, which considers the metallurgical, tribological, ergonometric, environmental, operating conditions, nature of the materials and cost aspects.

Electro Spark Deposition

Electro Spark Deposition, or ESD, is a pulsed-arc micro-welding process for overlaying a metallic substrate with a conducting material. Short-duration, high-current electrical pulses deposit small amounts of the electrode material on a metallic substrate. The electrical pulses are typically three orders of magnitude shorter than in other pulse welding processes.

Pulse duration of a few microseconds combined with pulse frequencies in the 0.1 to 4-Khz range allows the substrate to dissipate heat during a duty cycle. This results in cooling rates that exceed 100°C/sec, depending on material. At this cooling rates nano-structures are generated, while in some alloys the structure may be amorphous.

The process generates no hazardous wastes, fumes or effluents, and requires no vacuum systems, chambers, chemicals or spray booths. The substrates require no special preparation and nearly any metal, alloy or cermet, which is an electrical conductor, can be applied to all metal surfaces.

Advantages of ESD

• The arc deposition of metal and/or cermet coatings is made directly onto metal surfaces with minimal surface preparation;
• Any electrically conductive material can be deposited on a metallic substrate;
• An alloy is formed between the substrate and electrode.
• Coatings of various thickness and compositions can be formed;
• No heat-affected zone is created in the base alloy. The coating does not change the bulk properties of the substrate;
• The coatings exhibits wear and erosion properties far superior to conventional coatings with high adhesive strength of the layer compared with coatings of similar chemistries;
• The energy input and the heat generated in the substrate material is low;
• The process is very simple, requires very low capital and skill allowing the surface engineer to use her ingenuity to develop appropriate applications.

The Coating System

The system comprises a capacitor-based power supply, an electrode holder (or applicator), and consumable electrodes. Electrical energy is stored in a capacitor bank, which discharges the energy across the electrode-substrate interface.

The applicator either vibrates or rotates the electrode pneumatically/ electrically. The frequency of vibration or the rotational rpm influences the roughness of the coating. At higher frequency and lower power relatively smoother coatings are produced. The equipment is easily maintained and transported. This allows the use of the installation in field conditions.

Application of ESD to Mandrels forSeamless Tube Processing

During the past 8 years, Technovations International Inc has introduced innovative applications of the ESD in metal manufacturing and processing. The applications fall in three categories:

1. Static conditions where the clamping tool holds a work piece with minimum relative motion;
2. Dynamic conditions where the tool is always in relative high speed motion with the work piece;
3. Conditions at high temperatures or stress where the relative motion between the tool and work piece is at lower speeds.

Applications in the third category are more challenging. These include:

• Mandrels for hot extrusion of seamless pipe and tubes;
• Mandrels for cold stretch forming of tubes;
• “Bots” or tooling for bending of pipes;
• Dies for forging.

Besides wear of the surfaces, some of these tooling are subjected to loss of strength at high temperatures. When the surface temperature of a mandrel increases, the yield strength of the tool steel in the surface zone is reduced significantly. The loss of strength can be correlated with the surface hardness of the mandrels. Measurement of hardness along the radius of a mandrel after it was removed from the service is shown in Figure 1.

Figure 1: Hardness profile in the mandrel

The mandrels are therefore cooled by water flowing in a central bore. Depending on the flow rate and the heat transfer conditions, the temperature near the surface of can reach a level that is above the service duty temperature of the material. At these temperatures due to phase transformation there is a significant loss of strength near the surface. The photomicrograph of this region as depicted in Figure 2, shows the trans-formation of the martensite near the surface of the mandrel and the consequent loss in the hardness and strength.

Figure 2: Transformed microstructure in the surface region

Even with a harder surface coating, the bulk material under the coating deforms and stretches long before the hard coating can reduce wear.

We have taken steps to ensure that with proper alloy selection and its heat treatment and proper cooling of the mandrels, the strength of the bulk material is maintained at the service temperatures. They are however, subjected to galling and adhesive wear. ESD coating on the critical wear area on these mandrels as shown in Figure 3 has reduced the wear of these mandrels.

Figure 3: An ESD Coated mandrel for hot extrusion

The results are summarised below:

The mandrels used earlier with conventional heat-treating gave 15-25 extrusions per mandrel.

Mandrels heat-treated with new heat treatment and cooling practice to ensure high strength at the service temperatures and with no overlay averaged 30 extrusions per mandrel.

ESD treated mandrels averaged 50 extrusions per mandrel.

Mandrels used in cold stretch reduction of the seamless tubes are coated with hard chrome to increase the surface hardness. The abrasive materials on the hot extruded pre-forms often scratch the hard chrome surface. Besides, environmental concern with hexavalent chromium has also intensified the search for a more benign alternative. Ground mandrels were coated with ESD in a set up shown in Figure 4.

Figure 4: ESD coating of a mandrel for cold stretch reduction of tubes

The deposition conditions were set to give a relatively smooth surface with 50 microns (2 mils) coating thickness. The surface was polished prior to use. The ESD coated mandrels gave a life in excess of two times the life of the hard chrome plated mandrels. The ESD coated mandrels are re-coated with regrinding while the chrome-plated mandrels have to be further processed to strip the chrome before regrinding. A heat treatment step to prevent hydrogen embrittlement is also required after chrome plating. The ESD coating has eliminated these extra-processing steps.

Summary and Conclusions

ESD is a relatively simple low cost surface engineering process, which has been used to develop solutions to wear problems in metal processing. Through systems engineering with the proper understanding of the metallurgical, tribological, and operational requirements, Technovations International Inc, has introduced several innovative applications that have prolonged the tooling life and reduced manufacturing cost.

Higher wear resistance has been achieved in severe conditions such as hot extrusion and cold stretch reduction of superalloy seamless tubes. In addition, workholding tools such as burnishing rolls have been restored by ESD. Wedges in tensile testing machines, gripper jaws in stretch levelling, hard chuck jaws, pusher pads, guide rolls and feed rolls have been coated to give longer tooling life. With the understanding of the wear mechanisms and ingenuity, ESD can be effectively used to reduce the cost of tooling in a tube and pipe plant.

Technovations International Inc
5 Betsy Lane, Littleton, MA 0460-5901, USA
Fax: +1 304 733 9358
E-mail: technovations2002@yahoo.com