Rivet joining is one type of permanent joining process that can join two metallic components sufficiently strongly. It provides a durable, sound and reliable joint; especially the anti-loosening capability even under incessant vibration favors riveting over welding in many occasions including bridge constructions. Riveting is basically lap joining of two parts using rivets with the assistance of strap plates. Rivet is basically a small cylindrical rod usually made of soft material with a head at one end. Cylindrical shank of the rivet must be sufficiently longer than cumulative thickness of components and strap plates. While riveting, these rivets are inserted through holes made on the components and the protruding end (tail portion) of the rivet is hammered (upsetting) to make another head. Such holes must be drilled on components prior to riveting at designed locations. Two fixed ends of the rivet grips the components firmly.
On the basis of temperature at which protruding portion of rivets are hammered, riveting can be grouped into two categories—hot riveting and cold riveting. In hot riveting, the rivet end is heated by some external means (like flame heating) before hammering. Heating temperature lies around 2/3rd of melting point of the rivet material. Due to such heating, the material becomes soft and plastic and thus lower upsetting force is required. So when rivet material is hard, like stainless steel, hot riveting is preferred as lower force is required. It is also favorable for large diameter rivets, usually diameter larger than 10mm. Thermal expansion of rivet due to heating also has important role in gripping strength.
On the contrary, cold riveting is performed at room temperature only. Here rivet is not heated and thus hammering is carried out at room temperature. So comparatively higher force is required for upsetting; however, no heat source is desired for heating rivets. Heating time is also not associated with it, so the process is comparatively faster. However, if rivet diameter is large or it is made of stronger material, then large amount of hammering force is desired. Various differences between hot riveting and cold riveting are given here in table form.
Table: Differences between hot riveting and cold riveting
| Hot Riveting | Cold Riveting |
|---|---|
| In hot riveting, protruding end of rivets are heated to an elevated temperature prior to hammering. | In cold riveting, hammering is carried out at room temperature. No heating is performed. |
| It requires a suitable heat source (like fuel or gas flame) for heating rivets. | No such heat source is required. |
| Heating a large number of rivets takes time. So hot riveting is time consuming process. | No heating time is associated with cold riveting, so it is faster process. |
| Because of volumetric shrinkage, tensile stress develops within rivets as it cools down. This stress helps gripping components tightly. | No tensile stress develops within rivets, so gripping is not very tight. |
| Here rivets are subjected to both shear and tensile force. | Here rivets are subjected to shear force only. |
| Due to tight gripping, hot riveting mostly provides leak-proof joints. | Cold riveting may not necessarily leak-proof. |
| Much lower force is required during upsetting as rivet remains in plastic state due to heating. | Comparatively higher force is required for upsetting as rivet material remains in room temperature. |
| Hot riveting is preferred either when rivet is made of ferrous metal or when rivet diameter is more than 10mm. | Cold riveting is preferred when rivet is made of soft materials or its diameter is smaller than 10mm. |
Heating the rivets and heat source: Hot riveting is performed by heating the protruding end of rivets to an elevated temperature (50 – 70% of melting point of rivet material) and thereafter upsetting that end to make another head. Thus it requires a pointed and high energy density heat source to quickly apply heat. Usually fuel or gas flame is utilized for such purpose. Cold riveting, however, does not require any heating for hammering and thus heat source is irrelevant to it.
Riveting time: Hot riveting takes more time due to heating of rivets just prior to hammering (upsetting). Cold riveting is performed at room temperature, so heating time is not associated to it. Thus it is faster, economic and more productive.
Stress on rivets, tightness and leakage: Rivet joint always experiences shear stress as force is usually applied along the components or perpendicular to the rivet axis. It is irrespective of the way riveting is performed (hot or cold). In fact, shear failure of rivets is important design criteria while designing rivet locations. Apart from shear stress, every rivet is also subjected to tensile stress when hot riveting is performed. This tensile stress is independent of external load on components. Like every material, rivets material also undergoes volumetric expansion during heating. Since free end of rivet is hammered at hot condition, so free contraction is restricted and thus tensile stress develops when it cools down. This tensile stress helps gripping components tightly in a leak-proof way. However, no such tensile stress is induced within rivets when cold riveting is performed as no heating is performed and thus no volumetric expansion or contraction is associated with it.
Hammering force: Engineering materials can be transformed from elastic state to plastic state either by applying sufficient force or by increasing temperature to certain level. A solid material in plastic state becomes very ductile and requires significantly smaller force (stress) for strain (change in dimension). In hot riveting, the rivet tail becomes plastic due to heating at elevated temperature and thus smaller hammering force is required. In cold riveting, the rivet material remains in elastic state as no heating is performed and thus comparatively higher hammering force is desired to bulge the tail.
When hot riveting is desired? If rivet material is hard, like stainless steel, then hot riveting is preferred otherwise large hammering force has to be applied. Consequently when rivet material is soft, like aluminum or brass, then cold riveting can be performed. From dimension point of view, if rivet diameter is larger than 10mm then hot riveting is recommended as it will require lesser force during upsetting. For smaller diameter rivets, cold riveting can be carried out.
Scientific comparison among hot riveting and cold riveting is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.
- Difference Between Hot Riveting and Cold Riveting by difference.minaprem.com.
- Introduction to Machine Design by V. B. Bhandari (2017, McGraw Hill Education India Private Limited).
