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3 标记制度
3.1 公称高度≥0.8D（螺纹有效长度≥0.6D）的螺母
公称高度≥0.8D（螺纹有效长度≥0.6D）的螺母，用螺栓性能等级标记的第一部分数字标记；该螺栓应为可与该螺母相配螺栓中性能等级最高的（表2）。
表2 公称高度≥0.8D螺母的标记制度
螺母性能等级 | 相配的螺栓、螺钉和螺柱 | 螺母 | ||
1型 | 2型 | |||
性能等级 | 螺纹规格范围，mm | 螺纹规格范围，mm | ||
5 | 3.6、4.6、4.8 | ≤39 | ≤39 | / |
5.6、5.8 | ||||
6 | 6.8 | ≤39 | ≤39 | / |
8 | 8.8 | ≤39 | ≤39 | ≤16 |
10 | 10.9 | ≤39 | ≤16 | ≤39 |
12 | 12.9 | ≤16 | / | ≤16 |
注：一般来说，性能等级较高的螺母，可以替换性能等级较低的螺母。螺栓-螺母组合件的应力高于螺栓的屈服强度或保证应力是可行的。 | ||||
由于超拧，螺纹组合件可能产生下列失效形式：
a）螺轩断裂；
b）螺杆的螺纹脱扣；
c）螺母的螺纹脱扣：
d）螺母和螺杆的螺纹脱扣。
螺杆的断裂是突然发生的，比较容易发现，而脱扣是逐渐发生的，就很难发现并增加了因紧固件失效而造成事故的危险性。
所以，对螺纹连接的设计，总希望失效形式是螺杆断裂。但由于各种因素（螺母和螺栓的材料强度、螺纹间隙、对边宽度等）影响脱扣强度，故不能在所有情况下，都能保证获得这种失效形式。
螺纹直径为 8～39 mm 的螺栓或螺钉或螺柱，按表2规定选配适当性能等级的螺母，当拧紧到螺栓（螺钉或螺柱）保证载荷时，螺纹组合件不会发生螺纹脱扣。
然而，超过螺栓保证载荷的拧紧，时有发生，故对螺母的设计应至少保证在超拧10%时，螺纹组合件的失效是螺杆断裂，以警告使用者，装配操作不当。
注：有关螺纹组合件强度和螺母型式方面更详细的资料，见GB/T 3098.2附录A。
3.2 公称高度≥0.5D，而＜0.8D（螺纹有效长度≥0.4D，而＜0.6D）的螺母
公称高度≥0.5D，而＜0.8D（螺纹有效长度≥0.4D，而＜0.6D）的螺母，由两位数字标记：第2位数字表示用淬硬试验芯棒测出的公称保证应力的1/100（以N/mm2计）；而第一位数字“0”则表示这种螺栓-螺母组合件的承载能力比淬硬芯棒测出的承载能力要小，同时也比3.1条规定的螺栓-螺母组合件的承载能力小。有效承载能力不仅取决于螺母本身的硬度和两位有效长度，而且还与相配合的螺栓抗拉强度有关。表3给出了螺母的标记制度和保证应力。
表3 公称高度≥0.5D，而＜0.8D螺母的标记制度和保证应力 N/mm2
螺母性能等级 | 公称保证应力 | 实际保证应力 |
04 | 400 | 380 |
05 | 500 | 500 |
4 材料
表4规定了螺母各性能等级适用的材料。材料的化学成分应符合有关材料标准的规定。
表4 材料
性能等级 | 化学成分，% | ||||
C max | Mn Min | P max | S max | ||
51)、6 | / | 0.50 | / | 0.060 | 0.150 |
82) | 041) | 0.58 | 0.25 | 0.060 | 0.150 |
102) | 051) | 0.58 | 0.30 | 0.048 | 0.058 |
122) | / | 0.58 | 0.45 | 0.048 | 0.058 |
1）该性能等级可以用易切钢制造（供需双方另有协议除外），其硫、磷及铅的最大含量为：硫0.34%；磷0.11%；铅0.35%。 2）为改善螺母的机械性能，必要时可增添合金元素。 | |||||
性能等级为05、8（1型螺母）、10和12级螺母应进行淬火并回火处理。
5 机械性能
按第8章规定的方法进行试验时，螺母的机械性能应符合表5的规定。
表面缺陷应符合GB/T 5779.2的规定。
表5 机械性能
螺纹直径 | 性能等级 | ||||||||||||||
04 | 05 | 5 | |||||||||||||
保证 | 维氏硬度HV | 螺母 | 保证 | 维氏硬度HV | 螺母 | 保证 | 维氏硬度HV | 螺母 | |||||||
Min | Max | 热处理 | 型式 | Min | Max | 热处理 | 型式 | Min | Max | 热处理 | 型式 | ||||
8≤d ≤10 | 380 | 188 | 302 | 不淬火 | 薄型 | 500 | 272 | 353 | 淬火并 | 薄型 | 690 | 175 | 302 | 不淬火 | 1 |
10＜d ≤16 | |||||||||||||||
16＜d ≤33 | 720 | 190 | |||||||||||||
33＜d ≤39 | |||||||||||||||
螺纹直径 | 性能等级 | ||||||||||||||
6 | 8 | ||||||||||||||
保证 | 维氏硬度HV | 螺母 | 保证 | 维氏硬度HV | 螺母 | 保证 | 维氏硬度HV | 螺母 | |||||||
Min | Max | 热处理 | 型式 | Min | Max | 热处理 | 型式 | Min | Max | 热处理 | 型式 | ||||
8≤d ≤10 | 770 | 188 | 302 | 不淬火1） | 1 | 955 | 250 | 353 | 淬火 | 1 | 890 | 195 | 302 | 不淬火 | 2 |
10＜d ≤16 | 780 | ||||||||||||||
16＜d ≤33 | 870 | 233 | 1030 | 295 | / | / | / | / | |||||||
33＜d ≤39 | 930 | 1090 | |||||||||||||
螺纹直径 | 性能等级 | ||||||||||||||
10 | 12 | ||||||||||||||
保证 | 维氏硬度HV | 螺母 | 保证 | 维氏硬度HV | 螺母 | 保证 | 维氏硬度HV | 螺母 | |||||||
Min | Max | 热处理 | 型式 | Min | Max | 热处理 | 型式 | Min | Max | 热处理 | 型式 | ||||
8≤d ≤10 | 1100 | 295 | 353 | 淬火并 | 1 | 1055 | 250 | 353 | 淬火并 | 2 | 1200 | 295 | 353 | 淬火并 | 2 |
10＜d ≤16 | 1110 | ||||||||||||||
16＜d ≤33 | / | / | / | / | / | 1080 | 260 | / | / | / | / | / | |||
33＜d ≤39 | |||||||||||||||
注：最低硬度仅对经热处理的螺母或规格太大而不能进行保证载荷试验的螺母，才是强制性的；对其他螺母不是强制性的，是指导性的。对不淬火回火的，而又能满足保证载荷试验的螺母，最低硬度应不作为拒收依据。 1）D＞16 mm的螺母，可以淬火并回火，由制造者确定。 | |||||||||||||||
6 保证载荷
表6规定了细牙螺母的保证载荷值。其中，螺纹的应力截面积A按下式计算：
式中：d 2——外螺纹中径的基本尺寸，mm；
d 3——外螺纹小径的基本尺寸（d 1）减去螺纹原始三角形高度（H）的1/6值，即：
H —— 螺纹原始三角形高度（H = O.866 025P ）,mm；
P —— 螺距，mm；
π—— 圆周率，π= 3.1416。
表6 保证载荷值
螺纹规格 D×P | 螺纹的应力截面积 AS | 性 能 等 级 | ||||||||
04 | 05 | 5 | 6 | 8 | 10 | 12 | ||||
保证载荷（AS×SP）,N | ||||||||||
薄型 | 薄型 | 1型 | 1型 | 1型 | 2型 | 1型 | 2型 | 2型 | ||
M8×1 | 39.2 | 14900 | 19600 | 27000 | 30200 | 37400 | 34900 | 43100 | 41400 | 47000 |
M10×1 | 64.5 | 24500 | 32200 | 44500 | 49700 | 61600 | 57400 | 71000 | 68000 | 77400 |
M10×1.25 | 61.2 | 23300 | 30600 | 44200 | 47100 | 58400 | 54500 | 67300 | 64600 | 73400 |
M12×1.25 | 92.1 | 35000 | 46000 | 63500 | 71800 | 88000 | 82000 | 102200 | 97200 | 110500 |
M12×1.5 | 88.1 | 33500 | 44000 | 60800 | 68700 | 84100 | 78400 | 97800 | 92900 | 105700 |
M14×1.5 | 125 | 47500 | 62500 | 86300 | 97500 | 119400 | 111200 | 138800 | 131900 | 150000 |
M16×1.5 | 167 | 63500 | 83500 | 115200 | 130300 | 159500 | 148600 | 185400 | 176200 | 200400 |
M18×1.5 | 215 | 81700 | 107500 | 154800 | 187000 | 221500 | / | / | 232200 | / |
M18×2 | 204 | 77500 | 102000 | 146900 | 177500 | 210100 | / | / | 220300 | / |
M20×1.5 | 272 | 103400 | 136000 | 195800 | 236600 | 280200 | / | / | 293800 | / |
M20×2 | 258 | 98000 | 129000 | 185800 | 224500 | 265700 | / | / | 278600 | / |
M22×1.5 | 333 | 126500 | 166500 | 239800 | 289700 | 343000 | / | / | 359600 | / |
M22×2 | 318 | 120800 | 159000 | 229000 | 276700 | 327500 | / | / | 343400 | / |
M24×2 | 384 | 145900 | 192000 | 276500 | 334100 | 395500 | / | / | 414700 | / |
M27×2 | 496 | 188500 | 248000 | 351100 | 431500 | 510900 | / | / | 535700 | / |
M30×2 | 621 | 236000 | 310500 | 447100 | 540300 | 639600 | / | / | 670700 | / |
M33×2 | 761 | 289200 | 380500 | 547900 | 662100 | 783800 | / | / | 821900 | / |
M36×2 | 865 | 328700 | 432500 | 622800 | 804400 | 942800 | / | / | 934200 | / |
M39×3 | 1030 | 391400 | 515000 | 741600 | 957900 | 1123000 | / | / | 1112000 | / |
7 公称高度≥0.5D，而＜0.8D 螺母的失效载荷
表7指导性地给出了不同性能等级螺栓的失效载荷值。对性能等级较低的螺栓，预期的失效形式是螺栓螺纹脱扣，而对性能等级较高的螺栓，可预期为螺母螺纹脱扣。
表7 螺母最小脱扣强度对应的螺栓保证载荷的比率
螺母性能等级 | 螺母最小脱扣强度对应的螺栓保证载荷的比率，% | |||
螺栓性能等级 | ||||
6.8 | 8.8 | 10.9 | 12.9 | |
04 | 85 | 65 | 45 | 40 |
05 | 100 | 85 | 60 | 50 |
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