By H. P. Beck & A. R. Jackson
Article originally featured in the American Welding Society Welding Journal
If processed properly and according to code requirements, recycled slag can be a reliable and feasible alternative to buying new flux. Recycling fused slag from the SAW process is economical and environmentally friendly.
Submerged arc welding (SAW) slag is recycled by taking the fused part of the slag after welding and processing it in a manner that allows it to be reused for the same SAW operation. This slag recycling process has been around the welding industry for many years, and trial-and-error experimentation through the years has made it a reliable and accepted process. Two major reasons why a welding manufacturer would consider the use of recycled submerged arc welding slag are cost savings and the environment.
The cost of processing recycled slag is less than the purchase of new flux from the manufacturer. Many times this can amount to savings of 50% or greater. Savings can also be realized by eliminating the need to collect the slag and have it removed to an approved landfill. Environmentally, recycling slag minimizes the use of nonrenewable resources such as minerals, and it reduces the mass of material that must be sent to a landfill. It should be noted, though, that in most recycling processes there is some loss in weight, and not all the slag is processed into reusable flux. Also, there is magnetic separation during processing in which magnetic impurities are removed and disposed of as waste. An average for this loss is 25% of the total weight processed.
To realize all of the advantages of recycling, it is essential that the process is performed properly and according to the standards established by industry. Below are steps required for recycling slag as established by two standards setting organizations.
Requirements of AWS Code
The recycling of submerged arc welding slag for use in welding operations that are in accordance with the American Welding Society (AWS) Structural Welding Code, 01 .1-94, requires that each time the slag is recycled it be tested. All welding and testing presented in this report was performed by an independent third party who was neither a part of the welding manufacturer’s nor the slag processor’s organization.
The Structural Welding Code also states the frequency of testing shall be in accordance with the AWS Filler Metal Procurement Guidelines, AS.01-87. This publication states that the required testing shall be performed for each lot shipped. The testing required is in accordance with either the AWS Specification for Carbon Steel Electrodes and Fluxes for Submerged Arc Welding, AS.17-89, or the Specification for Low Alloy Steel Electrodes and Fluxes for Submerged Arc Welding, AS. 23-90. The testing examined here was in accordance with AWS AS.17-89. The test coupon was a 1-in. (25-mm) thick plate welded with backing bar using 5/32-in. (4.0-mm ) diameter electrode, and the AWS classification was EMl 2K.
The welding parameters used were 550 A direct current electrode positive (DCEP), 28 V, and 15 in./min (6.3 mm/s) travel speed. The test coupon was large enough to remove five Charpy V-notch samples, a chemical analysis sample and one all-weld-metal tension specimen. For complete details of the test coupon, welding parameters, location and number of test specimens refer to Specification for Carbon Steel Electrodes and Fluxes for Submerged Arc Welding, AS.17-89.
Objective of the Test
The results are from a manufacturer that welds daily. There was no attempt to control using the exact same slag from the previous recycling. The first time this slag was recycled, the welding manufacturer had generated the slag from new flux known by the trade name Lincoin 860.
Figure 1 shows the general arrangement of the Charpy V-notch specimens, the mass spectrometer specimen and the all-weld-metal tensile specimen. Table 1 gives the results of the chemical analysis performed on the recycled SAW slag.
Table 1 – Certification Deposit Analysis
|Element||AWS Requirements||First Recycle||Second Recycle||Third Recycle||Fourth Recycle||Fifth Recycle||Sixth Recycle|
Methods of Chemical Analysis
Chemical analysis for the first through fourth recycle was performed by the wet analytical method, and the location for analysis was the reduced section of the fractured all-weld-metal tensile specimen. Chemical analysis for the fifth and sixth recycle was performed by mass spectrometer taken from a lowdilution area of the weld test coupon.For carbon, manganese and silicon, the A5.17 specification states both a minimum and a maximum. As can be seen in Table 1, all values for the weld tests made from the recycled SAW slag fall within the ranges for carbon, manganese and silicon. As for sulfur, phosphorus and copper, which can be considered contaminants in the weld metal, the A5.17 specification states a maximum value. Table 1 shows that the levels of sulfur, phosphorus and copper in the weld tests made with recycled SAW slag fall below these maximums, and in most cases, well below.
Table 2 shows the results of the all-weld-metal tensile specimen taken from the weldc oupons welded with the recycled flux and also the results of radiographic testing of these same weld coupons. The radiographic tests of each weld coupon demonstrate the ability of the recycled flux to produce welds of radiographic quality.The welds are examined by nondestructive testing in accordance with The American Society for Testing Materials (ASTM) method El 42 Controlling Quality of Radiographic Testing. The quality level of inspection was 2-2T. The acceptance criteria was as defined in A5.17.
As Table 2 shows, the weld coupons made with SAW slag have the ability to produce sound welds when examined by the radiographic nondestructive testing method. All six welds coupons made with recycled SAW weld flux were acceptable in accordance with the radiographic acceptance criteria.
Table 2 – Certification Mechanical & Radiographic Results
|All-Weld-Metal Tensile||AWS Requirements Data (ksi)||First Recycle (ksi)||Second Recycle (ksi)||Third Recycle (ksi)||Fourth Recycle (ksi)||Fifth Recycle (ksi)||Sixth Recycle (ksi)|
|Yield Strength||58 min.||63.6||60.9||61.7||63.2||58.7||61.4|
|% Elongation||22 min.||32.5||33||32.5||31||31.9||29.7|
|% R of A||N/a||67.9||70.8||72.6||70.9||73.9||70.1|
|Radiography Results Method E142||See AWS A5.17||Passed||Passed||Passed||Passed||Passed||Passed|
Mechanical Properties Testing
The A5.17 specification states that the all-weld-metal tensile specimen shall have a minimum yield strength of 58,000 lb/in.2 (400 MPa) and a tensile strength of 70,000 lb/in.2 (480 MPa) minimum and 95,000 lb/in.2 (650 MPa) maximum. Table 2 shows that the minimum yield strength was exceeded and the tensile strength was within the acceptable range for each of the six all-weld-metal tensile specimens taken from the coupons made with the recycled SA welding slag.
The specification states that the minimum elongation in the 2-in. (50.8-mm) gauge length of all-weld-metal tensile specimen shall be 22%. There is no requirement defined for the reduction of area in the all-weld-metal tensile specimen, but it is reported in Table 3 because it was very easy to obtain since the specimen was already available. Table 2 shows that all tensile specimens made from the six weld coupons exceeded the minimum requirement of 22%. The reduction in area of the all-weld-metal tensile specimen is reported for information only.
The A5.17 specification states that five Charpy V-notch specimens shall be machined from the weld test coupon and tested at the temperature for the classification required. In evaluating the results, the lowest and highest values obtained are disregarded. Two of the remaining three values used to calculate the average must equal or exceed the specified 20 ft-lb and none of the remaining three may be less than 15 ft-lb (20 J).
Table 3 shows that of the Charpy V-notch specimens machined and tested from the six coupons welded with recycled SAW flux all met this criteria. The samples were tested at -20°F.
Table 3 – Certification Charpy V-Notch Test Data in Ft/Lb
|Charpy V-Notch Impact Values||First Recycle Results Ft/Lb||Second Recycle Results Ft/Lb||Third Recycle Results Ft/Lb||Fourth Recycle Results Ft/Lb||Fifth Recycle Results Ft/Lb||Sixth Recycle Results Ft/Lb|
Testing in Accordance with ASME Requirements
The recycling of submerged arc welding slag for use in welding operations that are in accordance with the American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel Code Section IX, Welding and Brazing Qualifications, requires that each time the slag is recycled it be tested. But as an alternative to the certification testing and classification as described above, a welding manufacturer may also choose to perform a welding procedure each time the SAW slag is recycled. The recycled flux is then considered an unclassified flux.
The procedure qualification used in this report was performed on 3/8-in. (9.5-mm) plate with a single V-groove and backing bar. The base metal is P No. 1 as designated by ASME Section IX. The welding was performed with a 5/32-in. (4.0-mm) diameter electrode (AWS EM1 2K) with direct current electrode positive.
The welding of the test coupon was performed with 100% recycled flux. The objective of this testing was to perform procedure qualification testing and meet all the criteria required by ASME Section IX for the intended application by the welding manufacturer. The requirements stated that the weld coupon be tested in the as-welded condition and that the tensile specimens, bend specimens and chemical analysis meet the requirements of ASME Section IX’s latest edition and addenda.
Figure 2 shows the general arrangement of the face and root bend specimens, the mass spectrometer specimen, and the reduced section tensile specimens as they were removed from the weld test coupon.
Testing for Mechanical Properties
The ASME Section IX requirements for the mechanical testing of this procedure qualification test called for two tensile specimens be removed and tested. Each tensile had to exceed the minimum requirement of 58,000 lb/in.2 (400 MPa) ultimate tensile strength. ASME Section IX also requires that four bend specimens be removed and tested in accordance with and to the acceptance criteria of ASME Section IX.
Table 4 shows the results of the tensile and bend tests of the seven procedure qualification tests run with the recycled SAW flux. It can be seen that for each of the seven procedure qualification tests performed, the tensile specimens far exceeded the requirement, and all fracture locations were in the base metal and ductile in nature. Also, Table 4 shows that all bend specimens were acceptable in accordance with ASME Section IX requirements.
Table 4 – Continuous Qualification Mechanical Test Results
|ASME Requirements||First Recycle||Second Recycle||Third Recycle||Fourth Recycle||Fifth Recycle||Sixth Recycle||Seventh Recycle|
|Tensile Test||58 ksi Min.||72.9/73.4 ksi||70.7/71.7 ksi||70.7/69.2 ksi||74.4/74.0 ksi||71.4/72.8 ksi||71.9/72.2 ksi||74.4/74.4 ksi|
|Type||Reduced Sec.||Reduced Sec.||Reduced Sec.||Reduced Sec.||Reduced Sec.||Reduced Sec.||Reduced Sec.||Reduced Sec.|
|Location||N/a||Base Metal||Base Metal||Base Metal||Base Metal||Base Metal||Base Metal||Base Metal|
|Blend Tests||Per QW-163||Passed||Passed||Passed||Passed||Passed||Passed||Passed|
Chemical Analysis Requirements
ASME Section IX also requires that a chemical analysis of the deposited weld metal be made of each procedure qualification test coupon. Since the objective is to have a mild steel weld deposit, the weld is required to meet the criteria of A-No. 1 in accordance with ASME Section IX. These requirements call for 0.15% carbon maximum, 1.60% manganese maximum and 1.00% silicon maximum.
Table 5 shows the results of the chemical analysis of the weld deposit for the seven tests made with the recycled SAW flux. In all seven procedure qualifications, it can be seen that the required chemical elements were below the maximum requirement of ASME Section IX. The chemical analysis of the procedure qualifications shown in Table 5 were performed by the wet analytical method for one through six. The chemical analysis of the procedure qualification for test seven was performed by mass spectrometer.
Since it’s easy to get the results for additional elements using mass spectrometer, recycle test seven was also analyzed for sulfur, phosphorous, and copper. It was found that the sulfur was 0.02%, phosphorus 0.019%, and copper 0.23%. In referring to Table 1, it can be seen that the levels of sulfur, phosphorus and copper would have been acceptable even with certification testing in accordance with AWS A5.17-89.
Table 5 – Deposit Analysis Continuous Qualification
|Element||ASME Values For A-No. 1 Deposits||First Recycle Results||Second Recycle Results||Third Recycle Results||Fourth Recycle Results||Fifth Recycle Results||Sixth Recycle Results||Seventh Recycle Results|
Based on the data presented here, the following conclusions appear logical:
- The requirements of the American Welding Society’s D1 .1-94, Structural Welding Code-Steel, for certification testing of recycled slag can be accomplish on a consistent and repeatable basis.
- The requirements of the American Society of Mechanical Engineers’ Boiler & Pressure Vessel Code Section IX, Welding and Brazing, for procedure qualification testing of recycled slag can be accomplished on a consistent and repeatable basis.
- Submerged arc welding slag recycling programs should not be entered into without testing to prove the recycled product will have no detrimental effect on the weld deposit.
- Welding fabricators must evaluate the test results, choice of new flux, and make a commitment to keep contaminants out of the slag to be recycled.
- Welding fabricators must be assured that they will only receive recycled flux from slag they have generated and kept from being contaminated.
- The recycling of SAW slag into SAW flux is a feasible alternative to buying new flux for SAW process users. Recycling provides economic benefits to companies and it allows them to be environmentally responsible.