STOODY -- 50 HARDFACING TIPS

DOWN-TO-EARTH SOLUTIONS FOR THE TOUGHEST WEAR PROBLEMS

Hardfacing is the most economical way to improve the service life and efficiency of wearing metal parts. However, choosing a premium quality hardfacing product does not by itself guarantee the desired result. The base metal, the type of wear, the welding process, and quality control in welding are equally important factors in achieving success. Following the guidelines below will insure that you achieve the maximum benefit from your hardfacing operation.

BASE METAL IDENTIFICATION

1)     Know the composition of the base material before welding (austenitic manganese steel, mild steel, carbon steel, wear resistant steel, AR plate, alloy steel, etc.).

2)     Austenitic manganese steel is tough, work hardens under impact, and has excellent impact properties. However, because it has only fair abrasion resistance, it should often be hardfaced for better overall wear resistance.

3)    The tough properties of manganese steel can be lost if the base metal temperature is heated continually above 6000F (3150C). Weld beads should be distributed so as to avoid concentrated and prolonged heat input in one area.

4)    Never use mild and low alloy steel electrodes to build up or weld manganese steel. The weld deposit will be brittle and may spall.

5)    When rebuilding austenitic manganese steels, use a manganese steel welding consumable. An austenitic (300 series) stainless steel weld product could be used but such weld deposits cannot be cut with an oxyacetylene flame, whereas manganese steel normally can.

6)     Preheating of plain carbon and alloy steels is often necessary to minimize distortion, cracking and spalling or to avoid thermal shock. Preheat temperature is influenced by carbon content and alloy content. The higher the carbon and, in most cases, the higher the alloy content, the higher the required preheat temperature. See Metals Preheating Charts in welding handbooks or Stoody Bulletin No.2112.

7)    Cast iron is extremely crack sensitive. Rebuilding and hardfacing of this base metal is not usually recommended because of the difficulty. However, some cast iron parts subjected primarily to straight abrasion are being hardfaced successfully. Under any circumstances, cast iron parts require thorough preheating at 8000F (4260C) or higher and must be cooled slowly after welding to insure success.

8)    In rebuilding manganese railroad track components it is desirable to remove about 1/8 in. (3.2mm) of a work-hardened surface before hard-facing or building up a worn area. Failure to do so might result in weld bead spalling.

WEAR MODE IDENTIFICATION

9)    Recognize and understand the wear problem (abrasion, impact, heat, friction, corrosion, vibration, compression, etc.). Singly or in combination they determine the choice of hardfacing alloy.

10)   A high deposit hardness does not always mean the best overall wear resistance. When selecting a hardfacing alloy for maximum wear resistance, consider the total alloy present as well as the presence of carbides.

ALLOY SELECTION

11)    Never make the assumption that higher priced alloys always provide the best wear resistance.

12)   If restoration is required prior to hardfacing, select a build up alloy that is both compatible with the base metal composition and the final overlay.

13)   The quality and hardness of the carbides in an alloy generally determines the deposit hardness and resulting abrasion resistance. Carbide percent in the alloy is the best indicator of abrasion resistance.

14)    Weld deposit hardness shown in the Stoody literature, unless otherwise specified, is normally based on two-layer weld deposits. A few specialty alloys are designed for multiple layers or for single layers only

15)    As a general rule, the higher the deposit hardness, the fewer build up layers can be applied.

Deposit Hardness          Number of Layers

       62-66 Rc                               1

       55-62 Rc                               2

       50-55 Rc                               3

       40-50 Rc                              4/5

       20-40 Rc                            Multiple

        The above is not applicable if the deposit has a fine, evenly spaced cross checking pattern and if the impact is low.

16)     Because of the presence of carbides, a two-layer hardfacing weld deposit with a Rockwell hardness of 50 Rc can outwear a single layer weld deposit with a higher hardness.

17) The hardness of a weld deposit can be determined by three different tests: Brinell, Rockwell, and Vickers. The three methods are related. One simple conversion is to multiply a Rockwell number by 10 to get the approximate Brinell hardness, or to divide a Brinell number by 10 to get Rockwell. Vickers is used to measure the hardness of the micro-constituents (carbides).

18) The weld deposit analysis shown in technical bulletins are, unless otherwise specified, all-weld-metal analysis; there is no amount of base metal dilution assumed in Stoody's technical bulletins.

19) A drastic loss in deposit hardness and wear resistance may occur if a wear resistant hardfacing alloy recommended for maximum 12000F (6500C) is used at considerably higher temperatures.

20) Iron-based, high carbon, high chromium alloys have very good abrasion resistance, moderate impact properties and moderate resistance to corrosion and heat. Use cobalt-based alloys for applications involving high temperature/corrosion.

21) The more wear resistant weld deposits, with higher alloy contents and hardness, have a tendency to "cross check" (form hairline cracks across the weld beads). A regular check pattern of weld bead deposition is desirable in many applications; it will reduce or even eliminate the tendency for distortion. Irregular crack patterns can result in spalling.

22) Tungsten carbide is one of the best metal to earth and fine particle abrasion resistant hardfacing materials. However; it should never be used above 10000F (5400C). A drastic reduction in wear resistance results from the oxidation of the tungsten carbide particles.

23) Where abrasion resistance is the primary requirement but deposit cost is a constraint, then the use of high chromium irons is suggested. In between high chromium iron and tungsten carbide is vanadium carbide, such as Stoody Van Car, which offers excellent abrasion resistance for the cost.

24) In determining the amount of welding electrodes required for a job, a rule of thumb is that 1/8 in. (3.2 mm) thick deposit on one square foot of base metal requires approximately 6 lbs. (2.7 kg) of weld-metal deposit; for tungsten carbide, increase the number to 8.5 lbs. (3.8 kg).

25) Some hardfacing wires are designed to eliminate interpass cleaning. Such alloys produce no slag and a second pass can be applied with no cleaning necessary.

WELDING PROCESS SELECTION

26) Select the most suitable welding process for the job (oxyacetylene, shielded metal arc, gas tungsten arc, submerged arc, plasma arc, open arc, thermal spray).

27) If maximum wear resistance is desired and only a single layer of surfacing material is applied, then the amount of base metal dilution is important. High base metal dilution can reduce wear resistance.

28) Dilution will vary with the welding process. The average percentage per pass for each process is as follows;

Oxyacetylene                   5%

Shielded Metal Arc         30%

Gas Metal Arc               20%

OpenArc                       20%

Gas Tungsten Arc          15%

Submerged Arc             40%

Plasma Arc                  20%

        Successive passes are diluted only by the deposit from the previous pass.

29) When preparing a job quotation, you should know how much weld metal you can put down per hour. On the average, deposition rates, at a 100 percent duty cycle, are as follows:

Type of Welding                Lbs.Mr.             Kg./Hr.

Oxyacetylene Welding            3-5                  1.4 - 2.3

Shielded Metal Arc                4-5                   1.8 - 2.3

Gas Metal Arc                    15 - 25                6.8 - 11.3

Open Arc                           15 - 30                 9.1 - 13.7

Gas Tungsten Arc                 4-5                   1.8 - 2.3

Submerged Arc                  18 - 35                 8.2 - 15.9

30)   The weld deposition efficiencies of two welding processes can be quite different. The efficiency of manual shielded metal arc welding is about 65 percent (accounted for by coating weight and electrode stubs). With semi-automatic or automatic gas metal arc welding, efficiency is increased to the 90 percentile range. This means that with a 100 lb. (45 kg) coil of filler metal wire, 90 lb. (40 kg) or more is deposited as weld metal.

31)   If 1/4 in. (6.4 mm) or larger electrodes are presently used on a given hardfacing application, it is probably time to consider switching to semi-automatic open arc or gas metal arc welding for better economy.

32)   In oxyacetylene hardfacing, it is recommended to use an excess of acetylene. The outer envelope should be three times the size of the inner cone. A carburizing flame lowers the surface melting temperature and protects the base metal surface from excess oxidation during hard-facing. Nickel base alloys require a neutral flame.

33)   Most self-shielding flux cored or tubular wires can be used with either DC constant current (CC) or DC constant potential (CP) power sources.  Constant voltage (CV) is another term for constant potential. Except for Stoody 103, submerged arc tubular wires can be used with either DC or AC power sources.

34)   If an electrode is recommended for either DC reserve or DC straight polarity, try DC straight polarity. The deposition rate is higher, the weld bead build up is better, and dilution percentage is less. You may see a little more spatter however.

35)   The size of the welding cable to be used, such as 1/0, 2/0, 3/0 or 4/0, depends on the amount of amperage and the length of welding cable needed. The quality of the welds that are being produced will be affected if the cable is too small (overheating) and/or too long (voltage drop).

36)   Submerged arc fluxes affect the deposit chemistry, Rockwell hardness, bead shape, and surface appearance. Test flux/wire combinations to assure required performance. No flux is absolutely neutral. Contact Stoody for recommendations.

37) Use new or correctly screened flux to prevent contamination of weld metal. Reused flux may affect deposit chemistry if not controlled properly.

38)   Excessive flux overburden and fines will result in trapped gas under the slag which in turn can cause porosity and affect surface finish.

39)   Excessively high interpass and preheat temperature adversely affects the slag removal characteristics of many fluxes.

QUALITY CONTROL

40)    Weld deposit porosity might be the result of long arc length, wide weld bead, overheating, dirty base material or, in semi-automatic gas metal welding, too much wire extension.

41)   When welding austenitic manganese steel, the weld beads should not be wider than 5/8 in. (16.0 mm) with a good convex crown. Flat deposits have a tendency to develop center bead cracks, while large, wide weld beads tend to increase heat build up which should be avoided.

42)     Never put a tough ductile weld deposit on top of harder, more brittle hardfacing deposit. Such deposits can spall. The hard weld deposit should always be on top of the softer one.

43)     Never make wide weld beads when welding steels that should not be overheated. Remember that the wider the weld bead, the slower the welding travel speed; the slower the travel, the higher the heat input into the base metal.

44) With the use of modem composite manganese steel covered electrodes (as compared to bare manganese filler metal rods) or alloy cored manganese steel wires, peening of the weld deposit might not be necessary.

45) In manual welding, the backhand welding technique is preferable. Forehand welding might produce porous, unsound weld deposits.

46) The work lead and ground clamp from the work piece back to the welding machine must be securely attached. A poorly attached work lead may result in bead porosity.

BEAD PATTERNS

47) Hardfacing weld beads spaced from 1/4 in. (6.4 mm) to 1-1/2 in. (12.7 mm) apart and against the flow of an abrasive material will improve the wear life and require fewer pounds of hardfacing alloy when the earth, sand, coal, etc. is relatively fined grained.

48)   Where larger pieces of ore, rock, slag, etc. are being handled, beads applied parallel to the flow of material act as runners, allowing the abrasive agents to ride on the hard metal and protect the base metal.

49)    Waffle or herringbone patterns work well in sand or dirt which tends to pack between hard metal beads to provide additional base metal protection. A dot pattern works well on base metals which should not be overheated during welding.

50)   In some applications hardfacing deposits applied to surfaces are vulnerable to spalling from side loads. In such cases, applying the hard metal into grooves improves resistance to impact and spalling.

To contact Stoody you may call 800-227-9333 or visit www.stoody.com