Cold heading quality steels and cold heading process


Cold heading quality steels and cold heading process

Cold heading quality (CHQ) Steel is the raw material which is used for the production of fasteners such as bolts, screws, nuts, rivets, nails, and other similar complex parts.  Traditionally fasteners have been manufactured using the thread cutting or by hot working method. But now the trend is moving towards using the cold working process to enhance productivity and to keep the cost down. It also provides good surface finish and dimensional accuracies to the fasteners.

Cold heading process

Cold heading process revolves around the concept of altering initial steel “blank” through force, using a series of tools and dies to change the blank into a finished product. The actual volume of steel remains unchanged, but the process maintains or improves its overall tensile strength. Cold heading is a high speed manufacturing process that relies on metal flow due to applied pressure as opposed to traditional metal cutting. It is a type of forging operation which is carried without the application of any heat. During the process material in the form of a wire is fed into the cold heading machine, cropped to length and then formed in a single heading station or progressively in each subsequent heading station. During cold heading load should be below the tensile strength, but above the yield strength of the material to cause plastic flow.

The cold heading process uses high speed automated “cold-headers” or “part formers.” This equipment has the ability of transforming a wire into an intricately shaped part with tight and repetitive tolerances using a tooling progression at speeds up to 400 pieces per minute.

The cold heading process is volume specific and the process uses dies and punches to convert a specific “slug” or blank of a given volume into a finished intricately shaped part of the exact same volume.

Advantages of cold heading operations are as below.

  • Design versatility

i)                 High strength parts from non heat treatable alloys

ii)                Cost effective compared to milling, machining, hobbing and chemical etching

iii)               High production rates

  • Metallurgical effects

i)                Grain flow which improves the finished part grain structure

ii)                Improves strength, hardness, toughness and fatigue resistance

  • Material savings

Choice of material for cold heading

In cold heading the mechanical properties of the final component are directly influenced by the metallurgical properties of the raw material. So a lot goes into the selection of the raw material for the cold formed components. In a nutshell the name says CHQ raw material but a lot of metallurgical aspects are to be considered while manufacturing and selecting the CHQ wires and slugs. Choice of wide range of materials is available for cold heading operation and it includes

  • Low and medium carbon steel
  • Alloy steel
  • Stainless steel
  • Non ferrous materials like copper and brass etc.

In case of steels, many grades and standards are used for cold heading operation. Cold heading quality (CHQ) steels are mainly produced from pure iron ore and selected steel scrap under controlled conditions, which ensures low content of residuals, uniform chemical composition and clean steel. Controlled rolling and cooling ensure optimal properties including homogeneous micro structure, uniform mechanical properties as well as good surface quality and close dimensional tolerances.

The starting material for cold heading operation is normally wire rods mostly in coil form. These wire rods are pickled, drawn into wires and annealed before it is subjected to the cold heading operation. Cold heading quality wire involves a particular raw material chemistry to withstand heading and forming operations. It also should possess excellent workability and surface quality for the cold working process to take place. Strict control is needed during wiredrawing, heat treatment, coating and inspection to ensure high quality wires with consistent tensile strength, uniform microstructure and improved forgeability. A typical process flow for the production of CHQ wires from wire rods is given in Fig 1.

cold heading process flow

Fig 1 Typical flow sheet for CHQ production process

 Applications of CHQ steels

Typical applications of CHQ steels are given below. The CHQ steel products are shown in Fig 2

  1. Automotive
  • Engine – Cylinder head bolt, piston rod and spark plug shell etc.
  • Steering system – Ball joint stud, tie rod pin, pinion etc.
  • Wheel – Hub bolt and nut, etc.
  1. Airplanes –  High tension bolts for wings
  2. Construction – Fastener bolts for construction purposes
  3. Bearing – Ball Bearing and roller bearing etc.
  4. Industrial machine – Fastener bolt, nut, screw, rivet and nipple etc.
  5. Electronic parts – Material for switch relay etc.

cold heading products

Fig 2 Products of CHQ steels