Elongation Requirements for Stainless Steel Rebar Presented by

Elongation Requirements for Stainless Steel Rebar Presented by: Suresh Gupta ASTM Committee Meeting A 01. 05 May 8, 2012

Purpose of Elongation • Elongation is a measure of ductility, which provides confidence that metals can be formed, without cracking or fracturing. • In rebar, it defines ductility so that bars can be bent without cracking. – If bars do not crack during fabrication, any excess elongation is irrelevant.

Alloy Characteristics With all other variables remaining same, • Austenitic Alloys have highest elongation. • Duplex Alloys have elongation in mid range • Ferritic Alloys have lowest elongation

Measurement Methods and Facts • % Elongation, when measured in a fixed gage length, e. g. 8 inches, is lower than when measured in a gage length, as a multiple of diameter, e. g. 4 X diameter (4 d). (when 4 d is less than fixed gage length). • Elongation is generally lower as diameter goes smaller. • This fact is clearly described in ASTM E 8, Appendix XI

ASTM E 8 Standard Test Method for Tension Testing of Metallic Materials Appendix XI – Factors Affecting Tensile Test Results • Comparison of elongation values determined using different specimens requires that the following ratio be controlled: Lo/(Ao)1/2 (XI. 1) Where: Lo = original gage length of specimen, and Ao= original cross-sectional area of specimen. XI. 4. 4. 1 Specimens with smaller Lo/(Ao)1/2 ratios generally give greater elongation. Note: (Ao)1/2 is proportional to diameter (d). Therefore, for same ductility, larger diameter samples result in greater elongation values, when measured in fixed gage length.

ASTM A 955 -9 a • Until revision 10 a, elongation was not required for – sizes # 3 -5 Grade 75, probably because it was known that it is unrealistic or not necessary. – Grade 40 for sizes 7 and larger because, in any condition it will meet the requirement, anyway.

% Elongation Requirements for Rebar Carbon and Alloy steels • ASTM A 615 requires 6 to 7% for Grade 75 • ASTM A 706 requires 10 to 14% for all Grades. Stainless Steels • BS 6744 requires 14% minimum in 5 d gage length for Grade 500 (72. 5 ksi). • ASTM A 955 requires 20% minimum in 8 inch gage length for all grades, all sizes.

Manufacturing & Process Control • Even with precise chemistry and process control, duplex stainless steels meet the elongation requirement very marginally. – If rebar is supplied in coils, machine straightening will cause work hardening and elongation will go down even further. – This may cause unnecessary dispute.

Rationale • If 6% elongation in carbon steel is sufficient • If 14% in 5 d is sufficient for stainless steel rebar per BS 6744 in Europe, – It should be sufficient for us as well. FYI • ASTM Specifications for plain smooth bars require elongation in 4 d

Proposal • It is therefore recommended that, – Using a foot note to Table 3, elongation requirements be lowered to 14% for Grade 75, • for all sizes of duplex grades and, • all grades of stainless steels under 1. 00 inch.