LIFTING WIRE FOR HOT-DIP GALVANIZING BY THOMAS LANGILL, Ph.D.

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1 LIFTING WIRE FOR HOT-DIP GALVANIZING BY THOMAS LANGILL, Ph.D. INTRODUCTION The purpose of the American Galvanizers Association s wire study was to determine the working load limit for support wire that is used for overhead lifting in a hot-dip galvanizing facility. The wire is used to support steel pieces during the process of hot-dip galvanizing by tying to a support fixture that is moved by overhead cranes through a series of cleaning baths and, eventually, into a molten bath of zinc at 850 F. The wire must be able to support the steel piece through alkaline and acid baths, as well as through the high temperature zinc bath. The support wire is used for only one circuit through the galvanizing process, so the calculation of a working load limit does not need to include a factor for multiple usages that was used for the steel chain working load limit. There have been tests of wire breaking strength but none of these previous tests have included the high temperature condition. The most common choice for wire is 9-gauge annealed wire since it has good strength and can be bent and deformed to tie into knots to hold steel material on lifting fixtures. The working load limit is based on the breaking strength of the wire in its use condition as well as a safety factor. The wire must be tied into a knot to hold the Figure 1 Tying for Light Gauge Material steel piece on the support fixture so the strength of the knot must be obtained. For larger parts, multiple wires may need to be used to support the weight. The steel part, including some portion of the wire, is first immersed into a bath of hot caustic solution. This is to remove paint, grease, oil, and soil from the surface of the steel to be galvanized. The next step is to immerse the steel part and the wire into an acid bath to clean the iron oxides off of the steel surface. The two acids used in the galvanizing industry are hydrochloric and sulfuric acid. Sulfuric acid baths are heated while hydrochloric baths are not. Once the steel part leaves the acid bath, it is rinsed and then immersed into a hot flux bath. The flux February 06 1

2 cleans the steel surface slightly and protects the steel surface until the part is immersed into the molten zinc bath. The zinc bath is held between 820 and 850 F. At these temperatures the wire will lose some of its tensile strength. The amount of tensile strength loss should be similar to the strength loss found during the testing of steel chains that are used in the galvanizing operation, a loss of 30% in tensile strength. The determination of a working load limit for lifting wire depends on the breaking strength of the wire at elevated temperature. The breaking strength could be a result of the wire breaking or the knot in the wire unraveling. This test program pull-tested and examined wires after they have been used in the galvanizing process to determine the breaking strength of these wires at elevated temperatures. Based on the test results, a working load limit was proposed to allow for safe use of lifting wire in the galvanizing operation. with a steel grade of AISI 1008 was used. The ultimate tensile strength of the wire is a maximum of 65,000 psi with a maximum breaking strength of 1123 lbs. The most important step in using wire ties is the knot that is formed when the material is attached to a lifting fixture. Hot-dip galvanizers use a number of different style knots for attaching steel parts to the fixtures. Three different wire tie techniques were studied and are shown below in Figures 3, 4 and 5. The technique that is most commonly used is shown in Figure 4. Figure 5 shows a technique used by one of the AGA members after review of his wire tying by his state OSHA office. Figure 3: Technique 1 Figure 2 Multiple Tying Figure 4: Technique 2 WIRE The wire most commonly used in the galvanizing process is 9-gauge. For the study, wire was purchased according to ASTM A 853, Steel, Carbon, for General Use. Black annealed, 9-gauge wire Twist once Twist twice Twist third time and bend back February 06 2

3 Figure 5: Technique3 To provide some cross-check on wire uniformity, two different wire manufacturers supplied wire for these tests. was purchased by two different galvanizers for the study. These two different galvanizers had different types of acid in their operation, verifying if the pickling acid had any effect on the wire strength through the process. Each wire sample used had to be at least 36 inches prior to tying so that the wire could be cut and then pull-tested after galvanizing to find the wire breaking strength. The lifting capacity suggested for the wires was 200 lbs to give some stress to the wire but to not overstress the wire during galvanizing. GALVANIZING PROCESS The wires were loaded with steel parts resulting in six samples for each tying technique at each galvanizing facility. The loaded wire ties ran through the entire galvanizing process: caustic, acid, preflux and zinc. The process variables for the two plants are given in Table 1 and Table 2. All samples at the Sulfuric Acid Plant (Plant A) were loaded with approximately 200 lbs before they were put through the galvanizing process. Personnel at the Hydrochloric Acid Plant (Plant B) were skeptical of the strengths produced by tying techniques 1 and 2, limiting the load on those wires to 38 lbs since they did not trust the breaking strength of these wires with higher loads. This plant had experience with wire tying technique 3, so the six samples for technique 3 were loaded with 205 lbs. After tying the wires they were all put through the same chemical cleaning and galvanizing process. Table1: Sulfuric Acid Plant Tank Temperature (F) Concentration Duration (minute) Caustic ounces / gallon 5 Sulfuric Acid % 41 HCl Acid (Rinse) % 7 Wet Kettle - Zaclon Top Flux February 06 3

4 Table2: Hydrochloric Acid Plant Bath Temperature (F) Concentration Duration (minute) HCl Acid 70 HCl 13.3 % 90 Fe 7.0 % Zn 1.0 % Rinse Water Preflux Zaclon K 95 Baume 12 degrees ph 5 Kettle 843 Technique 1 3 Technique 2 3 Technique 3 5 Following the galvanizing process the wires were cut to release the hot-dip galvanized steel parts and the wire was saved for pulltesting at elevated temperature. The wire samples were cut as indicated in Figure 6 keeping the knot portion intact to test both the wire breaking strength and the pullstrength of the wire tying techniques. The loop was cut on the opposite side of the knot so the knot would be preserved in the final length of the wire. Figure 6 Cutting Technique Cut wire tie at bottom keeping Knot is kept intact. using special fixtures designed for these wire tests. The length of the free ends of the wires varied significantly depending on the amount of wire used to form the knot. To be consistent, the free ends of each sample were trimmed before testing to a uniform length so that they fit smoothly in the test frame. The samples were heated in a furnace to 775 F. Each sample was assembled in the fixtures, the furnace placed around the samples, the furnace was closed and the heating begun. The temperature of 775 F was chosen to approach the melting point of zinc without going over the melting temperature so as to avoid changing the zinc coating on the wire. Each sample was allowed to equilibrate (soak) at temperature for 5-15 minutes before it was pull-tested. The furnace was then removed from around the wire and the wire was pull-tested. All samples were loaded at a standard rate of 1 inch per minute. Figure 7 Elevated Temperature System for Testing TEST PROCEDURE The 36 inch length of the wire was used to allow the test machine to grip both ends of the wire while leaving the knot in the center between the grips. All of the samples were pull-tested in a 5,000 pound MTS Q test load frame. The wire samples were tested February 06 4

5 TEST RESULTS The results of the pull-test at elevated temperature are documented in Table 3. There were six sample wires for each of the three wire tying methods as well as two wire manufacturers and two galvanizers. Each galvanizer used his own wire supplier. This matrix included 36 separate wire ties for testing. The wires were pulled apart by the MTS machine and the load recorded when the wire broke or the knot pulled apart. The table includes the and the. Many of the wires had their knots pull apart before the wire broke. This is recorded in the table as a failure description of wire untwisting. The averages for each wire tying method and each wire manufacturer are also reported in the table. Table 3 Pull-Test Results for High Temperature Testing Manufacture and Galvanizer A Sample Number Knot Strengths Knot Type 1 Knot Type 2 Knot Type Average B Average February 06 5

6 RESULTS DISCUSSION The results for the different wire tying techniques indicate significant data scatter. Another observation is that the results for Manufacturer A are consistently less than the results for Manufacturer B. Some of the difference in pull-strength can be attributed to the personnel at Galvanizer A who use wire tying method 3 as their standard practice and, therefore, do not use methods 1 or 2 except in this test. Galvanizer B can make some good wire knots using methods 1 or 2 but makes better knots using method 3, a method that is not regularly used at Galvanizer B. The other differences may be in the wire itself, either diameter or steel differences. Figure 5 shows the low values for pullstrengths when using either tying method 1 or 2. The values are significantly lower than tying method 3 pull-strengths. The same type of behavior is observed in Figure 6 for Galvanizer B but to a lesser degree. There are two very low pull-strength values, one for each tying method 1 and 2. When using this data to determine working load limits these low data points are going to heavily influence the final load limit. Galvanizer B had higher pull-strengths but the two low pull-strength numbers indicate that even with better attention to insuring the wires are tied tightly, wire tying methods 1 and 2 cannot be guaranteed to give high pullstrengths. tying methods 1 and 2 give much more erratic pull-strength values and consistently lower numbers than wire tying method 3. Figure 7 shows different results for the pulltests on the wires. Figure 7 shows the types of failures as a function of the wire manufacturer and galvanizer and versus the type of wire tying method. The high Figure 5 Pull-Test Results versus Sample Number Figure 6 Pull Test Results versus Sample Number Knot Strengths for Manufacturer A Sample Number Knot Strengths for Maunfacturer B Sample Number Knot Type 1 Knot Type 2 Knot Type 3 number of wire untwisting failures when using wire tying methods 1 and 2 indicates that the wires are not bearing against each other when using these two wire tying methods. The opposite is true of wire tying method 3 where it can be seen that almost all of the failures are due to the wire breaking before the knot becomes untwisted. This indicates that the wires are bearing against each other resulting in a knot that will not unravel during galvanizing. This will allow the wire tying method 3 knots to have a higher working load limit. The inconsistency of results for wire tying methods 1 and 2 needed to be investigated. A second phase of testing focused on the Knot Type 1 Knot Type 2 Knot Type 3 February 06 6

7 wire tying to explain the data inconsistencies. Figure 7 Breaking Type versus Tying Method Number of Samples A B A B A B Knot Type 1 Knot Type 2 Knot Type 3 Breaking Type PHASE 2 TESTS AND RESULTS Phase 2 testing consisted of a second set of samples, a total of 12 wires. The emphasis in this phase of testing was on the observation of the knot tying techniques and documenting the completed knots before galvanizing. In this phase, Galvanizer B performed the knot tying using method 1 and method 2 for 6 wire ties each. The knots were photographed before the galvanizing operation. After galvanizing, the knots were prepared in the same manner as in Phase 1 and the knots were sent to the labs for pull-testing. The MTS wire pull-test results are listed in Table 4. Table 4 Phase 2 Pull-Test Results for High Temperature Testing Manufacture B Sample Number Knot Strengths Knot Type 1 Knot Type Average The examination of the pictures and the physical observation of the wire tying indicated that the lower pull-strength numbers are from knots that are looser than the others. Knots that are pulled tight and have wire deformation in the knot have higher pull-strengths. This was true for both wire tying methods. Phase 2 results emphasize the need for deforming the wire during tying and the requirement of pulling the knot tight before putting the wire under load. February 06 7

8 Conclusions Working Limits (WLL) were developed as part of the study. The wire tie technique # 3 performed the best in the study. The WLL for technique 3 is based on the pull-strength numbers at the galvanizing temperature that averaged 551 and 669 pounds for the two sets of wire tests. Since the wire is used only one time for lifting loads the safety factor can be ½ of the high temperature breaking strength or 300 lbs for wire tying method 3. tying techniques 1 and 2 did not perform as well in high temperature tests. The wire knots were not as strong as with wire tying method 3, with a majority of the wire tying methods 1 and 2 tests ending when the knot became untied. It s believed that human skill and attention to detail play a large role in the performance of these knots. Since there is significant variation in the high temperature pull- strength values for these two tying methods, the WLL for both wire tying method 1 and 2 is 100 lbs. The lowest measured pull- strength for these methods was in the range of 200 pounds and with a safety factor of ½ this puts the working load limit at 100 pounds. The critical factors in determining the working load limit for wire use in hot-dip galvanizing operations are the high temperature use in the galvanizing kettle, the one time use of the wire, the deformation of the wire during tying, and the extra step of pulling the knot tight before applying a load to the wire. Any galvanizer can determine his working load limit by taking typical examples of his wire tying technique to a testing lab that can measure breaking strength of the wire near the zinc melting point temperature of 795 F. Once this pull- strength is established, then the working load limit can be calculated using a factor of ½ multiplied times the measured breaking strength for the one-time use of the wire tie. December

9 BIBLIOGRAPHY 1. Galvanizers Association of the United Kingdom, Galvanizing Manual, UK Galvanizers Association 2. Kleingarn, J-P, Guidelines on the use of lifting and transporting equipment in hot-dip galvanizing plants a contribution to accident prevention, VDF, Germany, 15 th Annual Intergalva Conference, Rome, Italy, June Occupation Health & Safety Administration, Code of Federal Regulations, 29CFR , Slings, March American Society of Testing & Materials (ASTM), A 853, Specification for Steel, Carbon, for General Use, Zalk Steel Report, Proper Use of Nine Gauge, Zalk Steel and Supply Company December