| HT-21 | Heat Treatment of Carbon and Alloy Steels up to 200,000 psi | Heat Treat |
| Supercedes: None | Superceded By: None |
| Number | Name | State | Country |
| 90203122 | Accurate Steel Treating Inc. | CA | United States |
| 90049129 | Bodycote Thermal Processing - Huntington Park | CA | United States |
| 90055923 | Bodycote Thermal Processing - Santa Fe Springs | CA | United States |
| 90049444 | CERTIFIED STEEL TREATING CORP | CA | United States |