Chapter 4 Surface Defects

In this section, the types, names, minimum allowable limits for appearance features, and acceptance inspection methods of surface defects on nuts are mainly described. We use D to represent the nominal size of the nut, DC to represent the flange diameter (maximum), S to represent the nominal (maximum) width of the opposite side, and the units of D, DC, and S are inches.

1、 Cracks:

（1） Quenching cracks:

Appearance features: Cracks are caused by transverse cutting of grains or along grain boundaries, and will produce obvious cracking marks along foreign impurities. Cracks are generally caused by overstress during heat treatment, forging, or other forming processes. The object is subjected to re treatment, and the heat treatment cracks often change color due to rust spots.

2. Allowable limit: No degree (depth, length) or position of cracks are allowed.

（2） Forging cracks and impurity cracks

Appearance features: Due to forging or cutting of rough blanks, forging cracks occur on the top surface and edge corners of the nut. Impurity cracks are caused by metal impurities or foreign objects already present in the material of the firmware. Typical forging crack impurity crack morphology.

2. Allowable limit: If there are no more than two cracks from the hole to the edge, no crack from the hole to the first tooth, and the crack depth does not exceed 0.5 times the tooth height, and the crack width does not exceed 0.02D or 0.012IN, then forging cracks and impurity cracks on the top and bottom surfaces of the nut are allowed.

（3） Locking crack:

1. Appearance features: The locking end of the torque nut is compressed during the forming process, and cracks often appear near the locking end, which may also be typical locking end cracks at the inner or outer edge.

Allowable limit: Locking cracks on the outer surface of the torque nut, provided that the nut can pass the torque test. It is allowed, if the nut can pass the torque test.

2、 Bursting and shearing

1. Appearance features: Burst is a crack formed by the internal explosion of metal, which occurs on the side or corner of the nut or on the edge of the flange nut during the forging process. Shear cracking is a type of crack that forms a 45 degree angle with the main shaft inside the metal, often occurring on the periphery of the flange nut. It is a typical form of cracking or shear cracking.

2. Allowable limit:

(1) For hexagonal nuts: If the burst or shear burst does not extend from the surface to the upper crown slope or to the bottom bearing surface, and the burst/shear burst occurs at the junction of adjacent heads/bottoms, with a width not exceeding 0.010 plus 0.02S, and the diagonal dimension is still within the lower tolerance limit, then the burst/burst is allowed.

(2) For flange nuts: If there are two or fewer bursts/shear bursts around the flange, with a width not exceeding 0.04DC, and one burst/shear burst has a width not exceeding 0.08DC, then burst/shear burst is allowed.

3、 Sewing

1. Appearance features: The seam often exists on the raw material and appears intermittently in a straight or smooth line along the axial direction of the nut during processing. After forging, it may cause cracking due to the seam. Typical seam pattern.

2. Allowable limit:

(1) If the seam is formed on the surface and the width does not exceed 0.02D, the seam is allowed.

(2) For hexagonal nuts with symmetrical call diameters ranging from 1/4 to 1/2 (inclusive), when the above-mentioned gap is generated, its guaranteed load should be tested. If it is within the standard range, the gap is allowed.

4、 Crease mark

Appearance features: The material wrinkling phenomenon generated during forging processing is called wrinkling, which often occurs on the top or bottom of the nut or at or near the diameter change, with a typical wrinkling pattern.

Allowable limit: If creases occur at the junction of the flange and the flange nut and extend to the main bearing surface, creases are not allowed. Other creases are allowed.

5、 Gap

1. Appearance features: The voids inside the raw material or the pores generated on the surface of the object after forging processing are called voids. The gap is formed due to rust spots on the raw material, chip burrs, or incomplete filling after mold marking and compression, and is a typical gap shape.

2. Allowable limit:

(1) If the depth of the gap generated on the surface of the nut does not exceed 0.010 inches or 0.02D, and the total area of all gaps does not exceed 10% of the minimum bearing surface area, then the gap is allowed.

(2) The method of determining the gap area on the bearing surface must be agreed upon by both the buyer and seller.

6、 Scratches on tools

Appearance feature: During the movement of the mold, shallow grooves resembling longitudinal or concentric circular grooves appear on the surface of the nut, which is called tool abrasion. A typical type of tool abrasion.

Allowable limit:

(1) If the surface roughness Ra of the bearing surface does not exceed 3.2UM, tool abrasion is allowed.

(2) Scratches caused by tools in other areas are allowed.

7、 Scratches and impact marks

1. Appearance features: Scratches and impact marks are caused by strong friction or collision between objects and manufacturing equipment during the manufacturing, operation, or transportation process, resulting in dents on the surface of nuts.

2. Allowable limit: Scratches, dents, teeth marks, and abrasions are allowed without compromising the functionality of the product.

8、 Overlap at the thread:

Appearance feature: Due to improper tooth rolling processing, there is a phenomenon of metal material wrinkling. This kind of partial overlap of tooth patterns is not allowed.