Thermal and Electrochemical Finishing

Thermal Deburring. Burrs and flash, both internal and external, are rapidly burned away using the thermal energy method (TEM). Parts to be processed are loaded in baskets or special fixtures and placed in a water-cooled furnace chamber. A fuel mixture of natural gas and oxygen is then injected into the chamber under pressure. The initial fire or heat is supplied by an ignition system in the form of a spark from an ignitor. The spark causes the combustible gas mixture to ignite, and all the fuel gas is consumed in approximately 2 ms to form a 3300 °C (6000 °F) heat wave.

The heat wave hits everything within the chamber. It hits the water-cooled walls of the chamber, the parent metal of the part or parts, and the burrs within blind holes, external edges, and intersecting holds (small or large) that cannot be reached by hand. The main body of the part becomes warm (usually under 150 °C, or 300 °F), while the flash or burrs (having less mass per surface area) heat up instantly and burst into flames. Heat created by the burning of fuel gas thus triggers the start of a second fire of the burr material itself. Because there is an abundance of oxygen in the initial fuel mixture, burrs will continue to burn until the heat is dispersed throughout the main body of the part.

Applications for the TEM include deburring of various steel castings and machined gears. For the gears, burrs are removed from the teeth and from tapped holes at the rate of 120 gears per hour. More detailed information on thermal deburring can be found in the article "Thermal Energy Method" in Machining, Volume 16 of the ASM Handbook.

Electropolishing is an electrochemical process for removing metal. Etching, deburring, smoothing, coloring, and machining are typical electropolishing processes. The removal of metal is done anodically in an acid or alkaline solution.

During the process, products of anodic metal dissolution react with the electrolyte to form a film at the metal surface. Two types of films have been observed: (a) a viscous liquid that is nearly saturated, or is supersaturated, with the dissolution products; and (b) anodically discharged gas, usually oxygen. Both types of films exist simultaneously in most commercial electropolishing solutions. The gas appears to be a blanket on the outside of the viscous film. Which type of film predominates depends on (a) the kind of metal, (b) the nature of the electrolyte, and (c) the surface condition prior to electropolishing (i.e., surface contamination, grain size, inclusions).

The most widely used electropolishing solutions for steels contain one or more of the concentrated inorganic acids--sulfuric, phosphoric, and chromic. Table 11 lists conditions for electropolishing alloy steels in acid electrolytes. Some of the steel products that undergo electropolishing are low-alloy (4130, 4140, and similar steels) automotive piston rings, crankpins, cotton-picker spindles, hand tools, gears, television chassis, and paper knives.

Table 11 Conditions for electropolishing steels in acid electrolytes

Type of metal (and product)

Purpose of treatment

Bath volume

Installed power

Current density

Polishing cycle,

Daily production

Operators

min

Area

L

gal

A

V

A/dm2

A/ft2

No. of parts

m

ft2

Sulfuric-phosphoric acid electrolytes

Carbon steels (job-shop work)

Brighten; deburr

1500

400

1500

12

25-40

250400

Varies

Varies

1

4140 steel

Prepare for chromium plate

3200

850

4000

12

15

150

10

700010,000

4575

500800

3

Sulfuric-phosphoric-chromic acid electrolytes

Carbon steel

Smooth; deburr

1500

400

30

300

2

5000

90

1000

1

4130 steel (tools)

Bright finish

950

250

1500

9

17.5

175

4

20005000

2

Steels are electropolished for one or more of the following purposes:

• Improve appearance and reflectivity

• Improve resistance to corrosion

• Prepare metals for plating, anodizing, or conversion coating

• Remove edge burrs produced by mechanical cutting tools

• Remove the stressed and disturbed layer of surface metal caused by the cutting, smearing, and tearing action of mechanical stock removal or of abrasive finishing

• Inspect for surface imperfections in cast, forged, or wrought metal

• Remove excess material as desired for milling metal parts

Effect on Fatigue Strength. Removal of not more than 25 pm (1 mil) on the diameter of steel fatigue specimens by electropolishing can lower the endurance limit from 10 million cycles without failure to failure at 100,000 to 120,000 cycles at 520 MPa (75 ksi) (Ref 5). The decrease in diameter is not responsible for the loss. Grinding and hand finishing to the same undersize has no adverse effect.

Static stress-strain values on electropolished specimens showed little or no scatter, and when the electropolished surface was rubbed with used 000 emery paper, the original value for fatigue limit was obtained. The mild treatment indicates that any detrimental effect of an electropolished surface results from removal of a compressively stressed skin. It has been observed that wet blasting raised fatigue strength of electropolished specimens to the level attained by polishing mechanically.

The lower fatigue strength of electropolished specimens appears to be because of removal of, or inability to produce, compressive stress in metal surfaces. Thus, electropolishing is comparable in effect to a stress-relieving anneal. For example, mechanical polishing of a chromium-vanadium steel produced a compressive stress of 350 to 520 MPa (50 to 75 ksi) at a depth of 0.02 to 0.05 mm (0.008 to 0.002 in.) below the surface. The stress was relieved by heating at 500 °C (930 °F) for 2 h, which resulted in lowering the fatigue limit from approximately 580 to 560 MPa (85 to 80 ksi). Similar treatment of a low-carbon steel lowered fatigue limit from approximately 730 to 630 MPa (105 to 90 ksi). Electropolishing lowered fatigue limit comparably.

Fatigue strength is not always lower after electropolishing. In an alternating torsion test, a nickel-chromium-molybdenum steel heat treated to 1450 MPa (210 ksi) had 34% higher fatigue strength after electropolishing a ground surface. The same steel heat treated to a lower tensile strength showed lower fatigue strength after electropolishing.

Because fatigue data for electropolished specimens show considerably less scatter than for mechanically polished specimens, electropolishing tends to show true fatigue value characteristic of a particular metal and metallurgical condition. Thus, any irregularly stressed surface can be removed by electropolishing, and a uniform compressive stress can then be applied by controlled working, such as shot peening (Table 10), wet blasting, or mild abrasive polishing.

Removal by electropolishing of 100 pm (4 mils) of metal from the surface of mechanically polished low-alloy steel containing 0.44% C, 0.61% Mn, 2.48% Ni, 0.82% Cr, and 0.48% Mo that was heat treated to a tensile strength of 1100 MPa (160 ksi) changed the residual surface compressive stress of approximately 170 to 200 MPa (25 to 30 ksi) to a tensile stress of 0 to 34 MPa (0 to 5 ksi). The fatigue limit was lowered about 28 MPa (4 ksi). The stress gradient was approximately 140 to 200 MPa (20 to 30 ksi) in 0.100 mm (0.004 in.) in the mechanically polished surfaces. The residual surface stress in mechanically polished specimens differed by approximately 200 to 240 MPa (30 to 35 ksi) from that of electropolished specimens with comparable surface roughness; fatigue limits differed by approximately 580 to 550 MPa (85 to 80 ksi). Variability of fatigue strength after certain mechanical operations and after electropolishing is shown in Table 12.

Table 12 Typical surface compression stress and fatigue strength of various carbon steels finished mechanically or by electropolishing

Finishing method

Fatigue strength, % of value for mechanical polishing

Depth of cold work

Surface compressive stress

mm

in.

MPa

ksi

Mechanical polishing

100

<0.050

<0.002

620

90

Electropolishing

70-90

None

None

None

None

Lathe turning

65-90

0.50

0.02

Milling

0.18

0.007

Grinding

80-140

< .25

< .01

760

110

Surface rolling

115-190

1.00

0.04

900

130

Shot peening

85-155

0.50

0.02

1030

Source: Ref 5

0 0

Post a comment