Selecting Filler Metals: Matching Strength Criteria

1 Key Concepts in Welding Engineering by R. Scott Funderburk Selecting Filler Metals: Matching Strength Criteria Introduction This column is the first of a series that In Table , A36 and A570 Gr. 50 are imum specified Filler metal properties will address topics related to Filler metal both listed in the Group I category. are more closely matched to the base selection. The focus will be on the con- Matching Filler metal is shown as metal, and the tensile Strength values cerns of design engineers, beginning both E60 and E70 electrode and are very similar (see Table 2). with Filler metal Strength . The Strength flux/electrode classifications. A36 and of weld metal vs. base metals may be A570 Gr. 50 have different minimum All of the preceding examples are defined as Matching , overmatching or specified yield and tensile strengths, considered Matching , although the undermatching.

2 This column will as do E60 and E70 Filler metals. degree of match is different. The address Matching Filler metal. Obviously, Matching cannot be as sim- ple as Matching the base metal Strength (see Table 1). Matching tensile What is strengths often do not Matching Strength ? While AWS calls the preceding result in Matching combinations Matching , clearly the What is Matching Strength Filler minimum specified weld metal proper- yield strengths metal? The AWS Standard ties are not the same as the minimum Welding Terms and Definitions does specified base metal properties. The common element is that the minimum not contain such a term, although it Matching combinations for AWS , specified tensile Strength of the Filler has been used for years. Matching Table , Group III materials provide metal is always the same as or greater Strength , on the surface, would seem some additional insight, where the min- than the minimum specified tensile to imply that the Filler metal will deposit weld metal of the exact Strength as (or Matching ) the base metal.

3 Codes Table 1. Filler /Base Metal Strength Comparison in AWS , Table , Group I. have tables with lists of Matching Filler metals, such as the AWS Base Metal Matching Filler Metal Structural Welding Code Steel, Table AWS , Table , , as do various Filler metal suppli- ers. A careful review of AWS , Group I E60, Fy = 48 ksi (330 MPa) E70, Fy = 58 ksi (400 MPa). Fu = 60 ksi (415 MPa) Fu = 70 ksi (480 MPa). Table , shows that the Matching electrodes do not deposit welds with Yield, Tensile, Yield Tensile Yield Tensile exactly the same Strength as the base ksi (MPa) ksi (MPa). metal, and in reality, this is not what is Weld is between Weld is between Weld is 12 ksi Weld is 22 ksi meant by Matching . 36 min. 58-80 2 ksi (15 MPa) 12 ksi (80 MPa). A36 (80 MPa) (150 MPa). (250) (400-550) greater to 20 ksi greater to 10 ksi greater greater (135 MPa) less (70 MPa) less Weld is 8 ksi Weld is 5 ksi A572 50 min.

4 65 min. Weld is 2 ksi Weld is 5 ksi (55 MPa) (30 MPa). Gr. 50 (345) (450) (15 MPa) less (35 MPa) less greater greater Welding Innovation Vol. XVI, No. 2, 1999. Table 2. Filler /Base Metal Strength Comparison in AWS , Table , Group III. cracking tendencies may be mini- mized. A common misuse of tables Base Metal Matching Filler Metal of Matching Filler metals occurs when AWS , Table , other options are never considered. E80, Fy = 68 ksi (470 MPa). Group III Fu = 80 ksi (550 MPa) Particularly for high Strength materials (>70 ksi [480 MPa] yield), under- Yield, Tensile, ksi (MPa). Yield Strength Tensile Strength Matching Filler metals may significantly ksi (MPa). reduce cracking tendencies. Weld is 3 ksi A572 Gr. 65 65 min. (450) 80 min. (550) Weld is equivalent (20 MPa) greater Weld is 8 ksi Weld is 5 ksi Actual vs.

5 Minimum A913 Gr. 60 60 min. (415) 75 min. (520). (55 MPa) greater (30 MPa) greater Specified Properties The traditional definition of Matching . Strength of the base metal. The com- Ultimately, Matching compares weld compares minimum specified proper- parison is of the minimum specified and base metal properties. However, ties, not actual properties. For most properties, not the actual properties of welds are not specified per se; Filler applications, this has proven to be the delivered steel, or of the deposited metals are. Thus, tables of Matching adequate, even though, based on weld metal. Since these are minimum products typically are called Matching actual properties of either the base properties, actual deposited welds on Filler metals, not Matching weld metals. metal or the weld, the weld may be the the actual steel will routinely exceed those values.

6 Joints Requiring A common misuse of Matching tensile strengths often do not Matching Filler Metal tables of Matching result in Matching yield strengths because the yield-to-tensile ratio for The need for Matching Filler metals is Filler metals occurs most hot rolled steels is lower than that dependent upon joint type and loading when other options are of most as-deposited welds. Therefore, condition. AWS , Table never considered a match of both yield and tensile Allowable Stresses in Nontubular Strength is improbable. However, for Connection Welds shows that match- higher Strength steels, the yield-to-ten- ing Filler metal is required for only one lower Strength element. For example, sile ratio typically approaches the val- combination of loading and joint type A572 Gr. 50 with Matching Strength ues for welds and provides for a closer tension loading of CJP groove welds, E70 Filler metal may have Matching , match of both the yield and tensile but is permitted for all other welds and undermatching or overmatching rela- strengths.

7 Table 3 shows the average loading conditions. Thus, a simple tionships, based on actual properties. yield-to-tensile ratio for all the base conclusion could be to always use metals contained in Groups I and III Matching Filler metal. However, this In theory, specified service loads and the corresponding Matching Filler may preclude better options such as would be limited to some percentage metals of the AWS Code, undermatching combinations where of the minimum specified yield or ten- Table The difference between the Filler metal and base metal yield-to-ten- sile ratio is much less of the higher Table 3. Varying yield-to-tensile ratios prevent Matching both the yield and Strength combination (Group III) than tensile strengths (data from AWS , Table ). that of the mild steel combination (Group I) as shown by the percent dif- Base Metals Matching Filler Metals ference (% Diff.)

8 Avg. Fy/Fu* Weld Fy/Fu* % Diff Group I..62 E60 .80 22%. (mild steel). E70 .83 25%. Group III..80 E80 .85 6%. (higher Strength ). *Based on minimum specified values Welding Innovation Vol. XVI, No. 2, 1999. sile Strength . If this were the case, the Table 4 may be unacceptable. Further requirements. For an application weaker component in the system definition of Matching properties as a where E80 is required, the E70 prod- would not limit the design even at the function of the actual materials may be uct could be used, providing there is maximum design load. necessary. adequate assurance that the deposit- ed weld metal will still deliver E80. This is not necessarily the case for For high Strength properties given variability in the pro- welded components that are expected duction of the Filler metal, as well as to be loaded into the inelastic range.

9 Materials under- differences in procedures. Examples would include components Matching Filler metals in buildings subject to inelastic (plastic). deformations in large earthquakes, and may reduce cracking The yield and tensile Strength proper- ties for the base and weld metal are all roll-over protection devices on con- tendencies determined by standard tensile test struction equipment. Under these coupons, uniaxially loaded, slowly severe loading conditions where yield- It is sometimes desirable to evaluate strained, smooth specimens. Under ing is expected, it is preferred that such actual, or typical, properties of base different conditions of loading, and deformations be distributed throughout metals and Filler metals. For example, with different geometries, these the base metal, and therefore, the an electrode classified as an E70 mechanical properties will vary, gener- undermatching combination shown in (such as E71T-1) may also meet E80 ally resulting in higher yield and tensile strengths and reduced ductility.

10 Table 4. Matching (M), Undermatching (U) and Overmatching (O) tensile Strength combinations for A572 Gr. 50 with E70 Filler metal. Conclusion Matching Strength is not formally E70 Filler Metal - Strength Levels defined by AWS. However, the Base Metal - A572 Gr. 50 accepted interpretation is that the Filler Minimum Medium High metal tensile Strength will be equal to 70 ksi (480 MPa) 80 ksi (550 MPa) 90 ksi (620 MPa) or greater than that of the base metal. The need for Matching Filler metal is Min. - 65 ksi (450 MPa) M O O dependent upon the joint type and loading condition, and it is generally Strength required for CJP groove welds in ten- Med. - 80 ksi (550 MPa) U M O sion applications. Matching can be used for most applications, but in High - 90 ksi (620 MPa) U U M some cases, it may not be the most economical or conservative choice.