和A2一样，A4是公制紧固件的材料等级的代号，是螺栓、螺钉和螺柱的不锈钢组别和性能等级的标记。
标记方式如下：
下面是这些标记相对应的美国标准钢号以及化学成分：
等级标记 | AISI | 化学成分(单位%，除非注明，一般指最大值) | ||||||||
C | Si | Mn | P | S | Cr | Mo | Ni | Cu | ||
A1 | SS 202 | 0.12 | 1 | 6.5 | 0.2 | 0.15-0.35 | 16,0 - 19,0 | 0.7 | 5,0 - 10,0 | 1.75-2.25 |
A2 | SS 304 | 0.08 | 1 | 2 | 0.05 | 0.03 | 15,0 - 20,0 | 0 | 8,0 - 19,0 | 4 |
A3 | SS 321 | 0.08 | 1 | 2 | 0.05 | 0.03 | 17,0 - 19,0 | 0 | 9,0 - 12,0 | 1 |
A4 | SS 316 | 0.08 | 1 | 2 | 0.05 | 0.03 | 16,0 - 18,5 | 2,0 - 3,0 | 10,0 - 15,0 | 4 |
C1 | SS 410 | 0.09-0.15 | 1 | 1 | 0.05 | 0.03 | 11,5 - 14,0 | - | 1 | - |
C3 | SS 431 | 0.17-0.25 | 1 | 1 | 0.04 | 0.03 | 16,0 - 18,0 | - | 1,5 - 2,5 | - |
C4 | SS 416 | 0.08-0.15 | 1 | 1.5 | 0.06 | 0.15-0.35 | 12,0 - 14,0 | 0.6 | 1 | - |
F1 | SS 430 | 0.12 | 1 | 1 | 0.04 | 0.03 | 15,0 - 18,0 | 0 | 1 | - |
