Types And Causes Of Surface Defects On Fastener Nuts

Nuts are core matching connecting parts for threaded fasteners. During production and application, nuts are prone to various surface defects, including cracks, shear bursts, bursts, fissures, folds, dents, tool marks and mechanical damage. The cracks can be further divided into quenching cracks, forging cracks, inclusion cracks, cracks on the locking portion of all-metal prevailing torque lock nuts, and washer seat cracks of nut-washer assemblies. This paper elaborates on the definition, causes and inspection specifications of various surface defects on nuts.

1. Cracks

A crack is a crystalline fracture defect with a clear opening morphology, propagating along or across metal grain boundaries and usually containing foreign inclusions. Most cracks are induced by excessive stress during forging, forming and heat treatment processes, while some originate from inherent defects in raw materials. During secondary heating of workpieces, oxidation scale peeling often causes color variation at crack positions, making the defects more identifiable.

1.1 Quenching Cracks

Causes: During heat treatment and quenching, excessive thermal stress and structural strain lead to cracking. Quenching cracks distribute randomly with irregular orientations on nut surfaces.

Technical Requirements: Quenching cracks of any depth, any length and at any position are prohibited.

1.2 Forging Cracks and Inclusion Cracks

Causes: Forging cracks occur during blanking and forging processes, mostly located on the top and bottom surfaces of nuts or at the transition between end faces and flat sides. Inclusion cracks are caused by non-metallic inclusions inside raw materials, belonging to cracking induced by inherent material defects.

Technical Requirements: Cracks on the bearing surface, top and bottom surfaces of nuts shall comply with the following specifications:

a) No more than two forging cracks penetrating the bearing surface, with a maximum depth not exceeding 0.05D;

b) Cracks extending into the threaded hole shall not exceed the first complete thread;

c) The crack depth within the first complete thread shall not exceed 0.5H1.

Where: D = nominal thread diameter; H1 = actual thread height, H1=0.541P; P = thread pitch.

1.3 Cracks on Locking Portion of All-Metal Prevailing Torque Lock Nuts

Causes: Such cracks are generated during chamfering, forging or closing and flattening processes, appearing on either the outer or inner surface of the nut locking section.

Technical Requirements: Cracks produced by forging on locking portions shall meet the mechanical and functional performance requirements of nuts and satisfy the following criteria:

a) No more than two cracks penetrating the top circumference, with a depth not exceeding 0.05D;

b) Cracks extending into the threaded hole shall not exceed the first complete thread;

c) The crack depth within the first complete thread shall not exceed 0.5H1.

Cracks caused by closing and flattening on locking portions are strictly prohibited.

Where: D = nominal thread diameter; H1 = actual thread height, H1=0.541P; P = thread pitch.

1.4 Washer Seat Cracks of Nut-Washer Assemblies

Washer seat cracks refer to fractures at metal edges and raised riveting positions during the assembly and riveting of nuts and washers.

Causes: During washer riveting, uneven or excessive pressing force on the edges and raised positions of the nut results in cracking at the washer seat.

Technical Requirements: Washer seat cracks shall be confined within the flanged edges and raised areas after riveting. The washer shall rotate freely without jamming and shall not fall off.

2. Shear Bursts

Shear bursts are surface cracking defects formed during nut forging.

Causes: During forging, shear stress causes surface cracking on the outer surface of nuts or the circumference of flange nuts. Shear bursts generally distribute at approximately 45° to the nut axis.

Technical Requirements: Shear bursts on the flat sides of hex nuts shall not extend to the bearing surface; shear bursts on the circumference of flange nuts shall not extend to the top circumference. Shear bursts on diagonal edges shall not reduce the diagonal width below the specified minimum dimension. The width of shear bursts at the transition between top/bottom surfaces and flat sides shall not exceed (0.25+0.02s) mm. For flange nuts, shear bursts on the flange circumference shall not intrude into the minimum diameter range of the bearing surface, and the width shall not exceed 0.08dc.

Where: s = width across flats; dc = flange outer diameter.

3. Bursts

Bursts are surface cracking defects similar in morphology to shear bursts but different in formation mechanism.

Causes: Micro defects on the surface of raw materials expand during forging, forming surface bursts on the outer surface of nuts and the circumference of flange nuts.

Technical Requirements: If cracks caused by raw material defects are connected with bursts, up to 3 to 4 cracks may extend to the top circumference, while bursts are not allowed to extend. Bursts on diagonal edges shall not reduce the diagonal width below the specified minimum size. The width of bursts at the transition between top/bottom surfaces and flat sides shall not exceed (0.25+0.02s) mm. For flange nuts, bursts on the flange circumference shall not intrude into the minimum diameter range of the bearing surface, with a maximum width of 0.08dc.

Where: s = width across flats; dc = flange outer diameter.

4. Fissures

Fissures are narrow and shallow longitudinal surface defects formed by the cracking of material folds.

Causes: Fissures are inherent defects originating from raw materials, mostly formed during steel smelting and rolling processes.

Technical Requirements: The depth of fissures on all nut specifications shall not exceed 0.05D (D = nominal thread diameter).

5. Folds

Folds are surface metal overlapping defects caused by uneven material flow during nut forging.

Causes: During forging, uneven metal flow and material stacking at sectional transition positions, top and bottom surfaces form surface folds.

Technical Requirements: Folds at the transition between the flange circumference and bearing surface of flange nuts shall not extend onto the bearing surface; folds at other positions are acceptable.

6. Dents

Dents are shallow surface depressions caused by insufficient metal filling during forging and heading processes.

Causes: Surface rust, shear burrs and residual chips on raw materials are pressed onto the workpiece surface during forming and cannot be eliminated, eventually forming dents.

Technical Requirements: The dent depth h shall satisfy h≤0.02D and shall not exceed 0.25 mm at maximum. For area limitation: the total area of all dents on the bearing surface shall not exceed 5% of the total bearing area for nuts with nominal thread diameter D≤24 mm, and shall not exceed 10% for nuts with D＞24 mm. Where: D = nominal thread diameter.

7. Tool Marks

Tool marks are shallow linear grooves distributed longitudinally or circumferentially on nut surfaces.

Causes: Relative sliding and friction between cutting tools and workpieces produce regular shallow grooves on nut surfaces.

Technical Requirements: The surface roughness of tool marks on nut bearing surfaces shall not exceed Ra=3.2 μm (in accordance with GB/T 1031). Tool marks on other surfaces are permissible.

8. Damage

Damage refers to irregular surface indentations, depressions, scratches, notches and grooves formed after nut forming.

Causes: External collision, friction and extrusion during finishing, transportation, storage and assembly cause surface mechanical damage. Such defects have no fixed geometry, position or direction and are induced by external mechanical forces.

Technical Requirements: Damages that do not impair the mechanical properties and service performance of nuts are acceptable and shall not cause rejection. Optimized packaging and standardized transportation procedures can effectively avoid mechanical damage during transit.