Microstructure

Fig 4 depicts the metallurgical structure of a thermo-processed bar, both for the tempered martensite periphery and the ferrite-pearlite core. Also shown is the microstructure of an “As Rolled Bar” which is not subjected to thermo-processing. The difference in grain size is marked.

Micro-hardness

Fig 5 indicates the micro-hardness of a thermo-processed bar from the surface to the core.

The Properties

The basic aim of thermo-processing or ‘quenching & tempering’ technology is to meet the desired properties as per global civil engineering demand. Basically, these are:

1. Minimum Yield Strength            500 N/mm²
2. Minimum Tensile Strength         10% more than YS subject to minimum 560 N/mm²
3. Stress Ratio (TS/YS)                1.10 (generally 1.15 to 1.25)
4. Minimum A₅ Elongation            16 (generally 18 to 22)
5. Weldability                               Consistent with industry requirements.

This technology, as employed in Thermex^® installations, successfully permits production of rebars to meet international standards and codes of various countries. Thermex Systems do this in a very precise manner. Increased globalisation and large projects undertaken by multinationals in India makes it almost mandatory that quenching technologies used in the country satisfy these basic aims – meeting international & Indian specifications.

Germany adopted the newly developed “quenching and tempering” technology in mid-eighties and immediately reaped great cost savings on account of use of Grade 500 rebars. See Chart 5.

It is estimated that India could have saved about Rs 8,000 crores (80,000 million) in the 15 year period from 1985 to 2000 if it had changed over to use of Grade 500 rebars as was happening in developed countries. This estimate is based on rebar prices of Rs 12,500 per tonne. The savings today can be imagined if one considers the current price of Rs 25,000 per T. This is the price we paid for the ‘closed market’ policies of the past. The cost in terms of safety cannot be quantified.