Heavy melting steel (HMS) or heavy melting scrap is broken up into two major categories: HMS 1 and HMS 2, where HMS 1 does not contain galvanized and blackened steel, whereas HMS 2 does. HMS is typically traded as a blend of 1 and 2, either a premium blend (80/20) or lower grade mixes (70/30) and (60/40). For example, HMS 1 and 2 (80/20) should consist of HMS 1 and 2 in the ratio of 80% (HMS 1) and 20% (HMS 2).

Each grade guarantees a minimum piece thickness: at least 1/4 inch (6.3 mm) for HMS 1, and 1/8 in for HMS 2

HMS 1 and 2 specifications as defined by ISRI (North America’s Institute of Scrap Recycling Industries):

• ISRI SPEC 200 (HMS 1) – Wrought iron and/or steel scrap 1/4 inch and larger in thickness. All pieces must be smaller than 60×24 in.
• ISRI SPEC 201 (HMS 1) – Same as ISRI 200 except pieces must be smaller than 36×18 in.
• ISRI SPEC 202 (HMS 1) – Same as ISRI 200 except pieces must be smaller than 60×18 in.
• ISRI SPEC 203 (HMS 2) – Wrought iron and/or steel scrap, black and galvanized, 1/8 inch and larger in thickness.
• ISRI SPEC 204 (HMS 2) – Same as ISRI 203 except pieces must be smaller than 36×18 in.
• ISRI SPEC 205 (HMS 2) – Same as ISRI 204 except it may contain automotive scrap, except for thin guage material.
• ISRI SPEC 206 (HMS 2) – Same as ISRI 205 except pieces must be smaller than 60×18 in.