Document Fragment View

Matching Fragments

i) For screw -

275 VPN

ii) For Barrel - 350 VPN The macro-hardness of three steel pieces designated as a0, a1 and b0, b1, b2 selected randomly from the packets A and B was also measured in VPN scale. The average value (of three readings) is as under:-

i) For a0 - 142 VPN
ii) For a1 - 150 VPN
iii) For a2 - 158 VPN
iv) For b0 - 230 VPN
v) For b1 - 157 VPN
vi) For b2 - 163 VPN 7.2 Chemical Analysis The drilled chips from the failed screw were collected and chemically analysed to determine the constituent elements present. The analysed chemical composition is given in Table-I. Table I Analysis of Chemical Composition Element Weight Percentage Carbon 0.37 Silicon 0.30 Manganese 0.54 Chromium 0.40 Molybdenum 0.17 Vanadium 0.10 Copper 0.15   The drilled chips of steel pieces designated as a0, a1 and b0, b1 from packets A and B respectively were also chemically analysed. The observed chemical composition is given in Table-II.
 
First Order of State Commission
4. On consideration of these reports, the State Commission concluded in its order dated 26.05.1995 (which was corrected by way of adding the pages left out inadvertently during typing, under the State Commissions order dated 28.06.1995) as under:
 
(a) The macro-hardness test conducted by CMERI shows that the macro-hardness of the screws is 275 VPN, the macro-hardness of the barrel is 350 VPN whereas the macro-hardness of the small bits/pieces varies from 157 VPN to 242 VPN only.
 

(b) It is thus clear to even a lay person that the principal conclusion of the CMERI that the screw did not fracture because of jamming by any foreign material was based on the absence of scratches on the surface of the affected screw or the barrel. This is, to put it mildly, an unscientific argument. The hardness index (VPN) of the extruder screw was 275 and that of the barrel 350, according to CMERIs own measurements. As against this, the CMERIs measurements of the VPN of five of the six steel pieces supplied to it by the surveyor ranged between 142 and 163, and only for the sixth, the VPN was 230. Even High School Physics would show that with such hardness levels (142-230), none of these steel pieces could have produced scratches on a screw with VPN of 275, leave alone on (the inner surface of) the barrel with VPN of 350. In its own discussion on this point, the CMERI report conceded, Hence, it is established that a material of lower hardness cannot make scratches over a material of higher hardness without any evidence of rub on the component of lower hardness. In our view, it is a pity that in its enthusiasm for lecturing on what should have been done by all others concerned, the NML report failed to notice and comment on the reasoning of the CMERI report even on this elementary point. Per contra, suffice it to add that the reports of both Shri Mukherjee and Shri Ray emphasised the absurdity of the conclusion of the CMERI on this point in the face of its own measurement values of VPN.

 

(c) (i) The CMERI report also observed that the chemical composition, hardness value and also the micro-structure of the supplied specimen of steel pieces selected randomly from the packets A and B did not confirm that all of the pieces were foreign materials. Rather out of four, two of them were very close to the screw material in respect of chemical composition, hardness and microstructure.

 

(ii) We are unable to see the basis of these conclusions. First, the hardness values of five of the steel pieces, as already noticed, ranged between 142 and 163, and only for the sixth, the VPN was 230. The VPN of the screw was 275 in fact, further down the report, the CMERI observed, in the context of optical microscopic examination of the fractured screw, the following, . three different phases of the microstructure at the fracture and near fracture zone from case to core, consisting a thin hard surface coating layer A (of micro-hardness about 720 VPN) and a thin martensitic peripheral layer B (of micro-hardness about 550 VPN) that extended into the original heat treated structure of fine tempered martensite C (of micro-hardness about 310 VPN). Thus, on hardness value alone, the screw material was way above the observed hardness values of the steel pieces.