EUROMET PROJECT

FINAL REPORT

1. Ref. No.: A 88/02

2. Subject Field: Mass (hardness)

3 Type of collaboration: Traceability in the field of hardness

4A. Partners: NPL-IMGC (institutions)

4B. CEC funded? NO

5. Participating countries: GB – It 6. Title: Transfer to IMGC-(It) of the traceability for UKAS-accredited laboratories and technical and scientific collaboration between NPL and IMGC in hardness field 7. Progress: NPL has been recently reconstituted the calibration service of hardness blocks, thanks to the realization of new primary hardness standard machines. The machines have been compared (on the most important hardness scales) with IMGC standards and the comparison has given positive results (compatibility of measurements between the two laboratories). Upon request of NPL, after 10 years the agreement for the transfer of the traceability to IMGC for UKAS-accredited laboratories is therefore closed officially. 8. Coordinator´s name: Alessandro Germak Address: IMGC-CNR, Strada delle Cacce 73, I-10135 Torino – Italy Telephone: +39.011.3977.367 Telefax: +39.011.3977.426 E-mail: A.Germak@imgc.cnr.it 9. Completion date: 2003-07-14 10. Coordinator´s signature:

11. Date: 2003-07-14

IMGC Rockwell intercomparison

Abstract This report details the work of a hardness comparison between NPL and IMGC covering the two major Rockwell and Superficial Rockwell scales, supported at NPL. Previous comparison work had been done which highlighted a problem on the Rockwell scales this has since been rectified. This comparison provides the final stage in confirming the improvements made were successful. It also forms part of the build up to the transfer of UK hardness standards from IMGC to NPL. Which will go ahead when both parties are happy with the comparability between our two services. The work compared the hardness standard machines from NPL and IMGC, the profiles used by both labs and also each labs indenters. With the results obtained, the differences between the NPL’s and IMGC’s Rockwell calibration services where identified along with the effect caused by the different indenters and machines. Notibable differences were looked into more thoroughly to identify the reason for any problem. With the results obtained and conclusions made NPL should be a step closer to transferring the UK national Hardness standard from IMGC.

Contents Introduction As part of the 1996-1999 DTI’s National Measurement System (NMS) Programme for Mass it was agreed that the UK national standards for hardness should be re-established to cover the highest priority Brinell, Rockwell and Vickers hardness scales. An NPL report [1] submitted on the completion of the contract MPU 8/45.4 “Provision of hardness standards” provides detail of the purchase and commissioning of hardness machines and ancillary equipment required to re-establish a UK hardness calibration service. It also reviews the main factors contributing to the uncertainty of hardness measurement and highlighted the need for improvement in the Rockwell hardness machine, in particularly the depth sensing technique. These improvements have now taken place, namely the depth sensing on the 1,5 kN hardness machine was replaced by a new laser interferometer system. Which appears to be successful. As part of the re-establishment of the UK national standards for hardness it is required that both IMGC and NPL are happy with they way in which their measurements of hardness compare. Previous comparison work has been completed with satisfactory results, in all but the Rockwell scale. Believed to be due to the problem in the depth sensing components of NPL’s hardness machine. Now with the new Laser interferometer depth-sensing component installed and running we are able to compare the currently supported Rockwell scales with IMGC. It was agreed to do comparisons on the HRC, HRB and superficial HR30N and HR30T scales, with IMGC using both their own standard indenter and NPL’s, so as to separate the errors caused by the differences in the indenters, and the machines. This comparison provides the final stage in confirming the improvements made were successful, and in moving closer to the hand over of the UK national standards for hardness from IMGC to NPL, by demonstrating NPLs measurement service is capable of obtaining comparable results to the Italians.

Protocol The work carried out at NPL was done using NPL’s 1.5 kN hardness machine (shown in Figure 1), which applies forces from 30 N to 1500 N Figure 1. NPL hardness machine.

The work carried out at IMGC was done using their deadweight hardness machine (shown in Figure 2). Figure 2. IMGC Hardness machine

The chosen scales to be compared between our Labs were HRC, HRB, HR30N and HR30T, as they all required the use of the laser interferometer depth measuring system. Five hardness blocks were provided for the scales:

Table 1. Hardness blocks used. Scale\Block serial No. E10288 T12418 N11032 T40813 T12538 HRC 25 45 65 HR30N 46 64 80 HRB 70 95 HR30T 63 80

For each calibration two indentations were made centrally on the blocks to settle the blocks and systems in. Five indentations were then made for each block calibration, which were used to take the measurements of the block hardness. The locations of the five indentations are as listed in Figure 3, and Tables 2-4. These locations were decided upon to give a homogeneous distribution on the surface of the block and to take into consideration the two sets of indentations made by IMGC (with their indenter and with the NPL Indenter). The NPL standard Rockwell indenter and 1/16th “ steel ball indenter. Along with IMGC’s equivalent indenters were used. All measured readings were recorded and logged in the results.

Table 2. IMGC Indentation locations (Common - NPL - Indenters)

HRC HRB HR30N HR30T A7 A7 A9 A9 B1 B1 B3 B3 C1 C1 C3 C3 D1 D1 D3 D3 E7 E7 E9 E9

Table 3. IMGC Indentation locations (IMGC - Indenters)

HRC HRB HR30N HR30T A1 A1 A3 A3 B7 B7 B9 B9 C7 C7 C9 C9 D7 D7 D9 D9 E1 E1 E3* E9

Table 4. Proposed NPL Indentation locations (Common - NPL - Indenters)

HRC HRB HR30N HR30T A2 A2 A4 A4 B8 B8 B10 B10 C8 C8 C10 C10 D8 D8 D10 D10 E2 E2 E4* E4

* (HR30N 46 Block: IMGC indented E4 instead of E3 so NPL indent is now E3, only this block though).

Figure 3. Indentation locations.

Results/Analysis

NPL-IMGC comparison

Table 5. NPL measurements

Rockwell scales: B, C, 30N, 30T

Final velocity [(80-100)%Fmax] =

Scale Hardness block Indenter NPL Measurements [HRB]

Expanded uncertainty of the single

meas.

Standard deviation

of the meas.

Mean value

Expanded uncertainty of the mean

value

Preload dwell time

U=0,2s

Full load application

time U=0,2s

Full load dwell time

U=0,2s

1 2 3 4 5 [HR] [HR] [HR] [HR] [s] [s] [s]HRC E10288 NPL 1922 25.76 25.82 25.60 25.90 26.01 0.18 0.15 25.82 0.23 3.0 7.0 5.0HRC T12418 NPL 1922 46.55 46.50 46.50 46.53 46.58 0.13 0.03 46.53 0.13 3.0 7.0 5.0HRC N11032 NPL 1922 64.29 64.33 64.15 64.32 64.36 0.19 0.08 64.29 0.20 3.0 7.0 5.0

HR30N E10288 NPL 1922 46.32 46.46 46.46 46.67 46.74 0.12 0.17 46.53 0.19 3.0 7.0 5.0HR30N T12418 NPL 1922 65.53 65.71 65.62 65.65 65.93 0.18 0.15 65.69 0.22 3.0 7.0 5.0HR30N N11032 NPL 1922 80.96 81.01 81.05 81.13 81.28 0.18 0.13 81.08 0.21 3.0 7.0 5.0

HRB T40813 NPL 64563 72.52 72.05 72.60 72.19 72.82 0.12 0.31 72.43 0.31 3.0 7.0 5.0HRB T12538 NPL 64563 96.86 96.74 96.78 96.89 96.86 0.19 0.06 96.83 0.20 3.0 7.0 5.0

HR30T T40813 NPL 64563 64.93 64.32 65.45 65.75 65.03 0.18 0.54 65.10 0.52 3.0 7.0 5.0HR30T T12538 NPL 64563 79.07 79.23 79.09 79.29 79.19 0.18 0.09 79.17 0.20 3.0 7.0 5.0

NPL-IMGC comparison Table 6.

Rockwell scales: B, C, 30N, 30T

IMGC measurements

Final velocity [(80-100)%Fmax] = 27 µm/s ± 5 µm/s

Scale Hardness block Indenter IMGC Measurements [HRB]

Expanded uncertainty

of the single meas.

Standard deviation

of the meas.

Mean value

Expanded uncertainty of

the mean value

Preload dwell time

U=0,2s

Full load application

time U=0,2s

Full load dwell time

U=0,2s

Indenter Dif

1 2 3 4 5 [HR] [HR] [HR] [HR] [s] [s] [s] [HR]HRC E10288 IMGC DKM 33871 25.52 25.43 25.38 25.51 25.57 0.15 0.08 25.48 0.16 3.0 3.9 5.0 -0.36 HRC T12418 IMGC DKM 33871 46.17 46.12 46.01 46.10 46.15 0.15 0.06 46.11 0.16 3.1 3.7 5.0 -0.42 HRC N11032 IMGC DKM 33871 63.92 63.97 63.87 64.02 63.96 0.15 0.06 63.95 0.16 3.1 3.8 5.1 -0.44 HRC E10288 NPL 1922 25.79 25.86 25.60 26.07 25.90 0.15 0.17 25.84 0.21 3.1 4.2 5.0 HRC T12418 NPL 1922 46.50 46.62 46.38 46.68 46.45 0.15 0.12 46.53 0.19 3.0 3.7 5.0 HRC N11032 NPL 1922 64.35 64.39 64.35 64.45 64.40 0.15 0.04 64.39 0.15 3.0 3.5 5.0

HR30N E10288 IMGC DKM 33871 46.27 46.14 46.09 46.28 46.30 0.20 0.09 46.22 0.22 3.0 4.4 4.7 -0.20 HR30N T12418 IMGC DKM 33871 65.53 65.51 65.37 65.32 65.51 0.20 0.10 65.45 0.22 3.1 4.3 4.8 -0.27 HR30N N11032 IMGC DKM 33871 80.83 80.67 80.68 80.57 80.81 0.20 0.11 80.71 0.22 3.0 4.2 4.9 -0.29 HR30N E10288 NPL 1922 46.35 46.43 46.24 46.63 46.45 0.20 0.14 46.42 0.24 3.0 4.1 4.9 HR30N T12418 NPL 1922 65.66 65.86 65.52 65.79 65.75 0.20 0.13 65.72 0.23 3.0 4.2 4.9 HR30N N11032 NPL 1922 80.93 81.10 81.01 80.87 81.11 0.20 0.10 81.00 0.22 3.1 4.1 4.9

HRB T40813 IMGC 73.14 72.99 73.22 73.07 73.19 0.25 0.09 73.12 0.26 3.0 2.7 5.0 0.34 HRB T12538 IMGC 97.52 97.36 97.30 97.26 97.35 0.25 0.10 97.36 0.27 3.0 2.4 5.0 0.24 HRB T40813 NPL 64563 72.54 72.58 73.06 72.68 73.03 0.25 0.25 72.78 0.33 3.0 3.1 5.0 HRB T12538 NPL 64563 97.10 97.17 96.99 97.18 97.17 0.25 0.08 97.12 0.26 3.0 2.6 5.0

HR30T T40813 IMGC 66.25 65.58 65.63 66.07 65.83 0.30 0.29 65.87 0.39 3.1 4.0 4.9 0.27 HR30T T12538 IMGC 79.21 79.25 78.92 78.95 79.19 0.30 0.16 79.10 0.33 3.0 3.8 4.8 -0.26 HR30T T40813 NPL 64563 65.31 65.56 65.69 65.64 65.79 0.30 0.18 65.60 0.34 3.0 4.0 4.8 HR30T T12538 NPL 64563 79.40 79.38 79.28 79.37 79.41 0.30 0.05 79.37 0.30 3.0 4.0 4.9

Figure 4.

IMGC-NPL Comparison on Rockwell scalesdifferences due to the machines (common indenter)

-0.9-0.6-0.30.00.30.60.91.2

20 40 60 80 100

Hardness value/HR

Diff

eren

ces/

HR

HRCHR30NHRBHR30T

Figure 5.

IMGC-NPL Comparison on Rockwell scalesinstitute indenter

-0.9-0.6-0.30.00.30.60.91.2

20 40 60 80 100

Hardness value/HR

Diff

eren

ces/

HR

HRCHR30NHRBHR30T

Figure 6.

IMGC-NPL Comparison on Rockwell scalesDifferences due to the indenter

-0.9-0.6-0.30.00.30.60.91.2

20 40 60 80 100

Hardness value/HR

Diff

eren

ces/

HR

HRCHR30NHRBHR30T

Figure 4, shows the difference between the NPL and IMGC measure hardness readings, when the NPL indenters we used by both. As can be seen HRC and HR30N give very comparable results all within 0.11 HR. The larger differences occur with the HRB and HR30T scale up to 0.5 HR. NPL generally seems to measure the blocks slightly softer than IMGC. If you compare the results of these measurements in the form of en-ratios, it gives the comparability of the NPL 1,5 kN hardness machine with the IMGC deadweight hardness machine. Table 7. Table 7. En-ratios (NPL indenter) IMGC NPL Difference between ~Hardness Uncert Uncert Scale NPL and IMGC of Block En Ratio HR HR HR30N 0.107 46.5 0.35 pass 0.24 0.19 -0.028 65.3 -0.09 pass 0.23 0.22 0.081 81.3 0.26 pass 0.22 0.21 HR30T -0.501 65 -0.81 pass 0.34 0.52 -0.196 80 -0.54 pass 0.30 0.20 HRC -0.024 25 -0.08 pass 0.21 0.22 0.004 46 0.02 pass 0.19 0.13 -0.098 65 -0.39 pass 0.15 0.2 HRB -0.345 72 -0.76 pass 0.33 0.31 -0.296 96 -0.90 pass 0.26 0.2

These en-ratios show that the comparability between NPL and IMGC is good; anything below 1 is a pass. HRB and HR30T show the worst en-ratios. Figure 5. Shows the difference between the NPL results and the IMGC results, with IMGC using their own indenters. As can be seen the difference increases to up to ~0.8HR, showing that the indenter is a major factor in the uncertainty of the measurements. These increased differences between NPL and IMGC push the en-rations outside the limit causing some scales to fail. From the previous chart we know that this is down to the indenters not the machines. Figure 6, shows the difference cause by the indenters, the previous results adjusted by original errors, showing that the NPL Rockwell cone indenter produces harder readings, compared to the IMGC indenter, while the Rockwell ball indenters have the reverse effect, producing softer readings of a similar magnitude. To look at the areas where there is a large error, we need to look at the differences between NPL and IMGC. These differences are obviously the indenter, but also the profile used to make the hardness test, because of the range permitted in the standard Comparing the two hardness profiles the only real differences are in the main force approach time with IMGC ranging from 2.5-4.5 seconds and NPL having a constant 7 seconds for all the scales. Looking at the sensitivity coefficient work, which has been done, this difference should have no real effect on the HRB scale. While the sensitivity coefficients for HRC will cause an error of from –0.03 HRC to -0.07HRC

for 20HRC-70HRC respectively, which is not a huge effect compared to the differences between NPL and IMGC. The only other difference was that IMGC use a fixed velocity of 27µm/s ±5 µm/s for the final 20% of force. At NPL this was not controlled, due to the nature of the NPL hardness machine, although it is possible to program the software to have that approach velocity it can cause problems with overshoot. The current profiles were programmed in, with minimising any overshoot to an acceptable level, being the main objective. Which leaves the final velocity outside the recommended 20-40 µm/s range noted in the standard. Assessing the velocities from the measurements taken shows this to be the case. (See Table 8.) Table 8. Velocity analysis of final 20% of F.

Average velocity over last 20% of F

Preload dwell timeU=0,2s

Full load application time U=0,2s

Full load Difference to IMGC (v)

dwell timeU=0,2s

Scale Block (mm/s) [s] [s] [s] (mm/s) HRC 25 0.0169 3.0 7.0 5.0 -0.0101 HRC 45 0.0136 3.0 7.0 5.0 -0.0134 HRC 63 0.0108 3.0 7.0 5.0 -0.0162 HR30N 46 0.0064 3.0 7.0 5.0 -0.0206 HR30N 64 0.0049 3.0 7.0 5.0 -0.0221 HR30N 80 0.0037 3.0 7.0 5.0 -0.0233 HRB 70 0.0197 3.0 7.0 5.0 -0.0073 HRB 95 0.0121 3.0 7.0 5.0 -0.0149 HR30T 63 0.0060 3.0 7.0 5.0 -0.0210 HR30T 80 0.0041 3.0 7.0 5.0 -0.0229

There are sensitivity coefficients for velocity on the HRC scale, this scale is not where the main problem is, and it is difficult to know what effect they could cause for the other scales. If they have effects in a similar way to those of HRC they would cause a shift in the correct direction, closing the gap of the difference between NPL and IMGC. To this end the profiles for the problem scales were re-written to control the final velocity fixing at 27µm/s. Measurement were repeated on the HRB scale.

Extra work done to identify problems The new indentations were made in the locations listed in Table 9. Table 9. Scale HRB 1 A8 2 B2 3 C2 4 D2 5 E8 Tables 10 and 11, shows the measurements and data from the calibrations of the HRB blocks with the new velocity controlled profile:

Table 10. 70 HRB Velocity controlled

Mean Hardness HRB Time of pre Load (s)

Time of ramp to F (s)

Speed over last 20% (mm/s)

Speed over last 19% (mm/s)

Time of major load hold (s)

1 72.37 3 7.04 0.0253 0.0268 5 2 71.492 3 7.08 0.0251 0.0268 5 3 72.56 3 7.05 0.0252 0.0269 5 4 71.987 3 7.05 0.0255 0.0269 5 5 72.382 3 7.06 0.0252 0.0268 5 Mean 72.158 3 7.06 0.0253 0.0268 5 Standard Deviation 0.427 0 0.02 0.0001 0.0001 0

Table 11. 95 HRB Velocty controlled

Mean Hardness HRB Time of pre Load (s)

Time of ramp to F (s)

Speed over last 20% (mm/s)

Speed over last 19% (mm/s)

Time of major load hold (s)

1 96.7 3 6.89 0.022 0.0271 5 2 96.759 3 6.85 0.0227 0.0268 5 3 96.632 3 6.85 0.0228 0.0268 5 4 96.817 3 6.86 0.0227 0.0268 5 5 96.838 3 6.85 0.0227 0.0269 5 Mean 96.749 3 6.86 0.0226 0.0269 5 Standard Deviation 0.085 0 0.02 0.0003 0.0001 0

As can be seen the velocity was measured for the last 20% and also the last 19%. This was due to the fact that a certain amount of deceleration was needed to prevent very large overshoot. This deceleration was programmed into the last 1%. The total velocity is still maintained within the 20µm/s minimum limit.

Comparing these results with those with the previous profiles shows that increasing the velocity cause the measured hardness to drop: 0.275 HRB for the 70 HRB block and 0.077 HRB for the 95 HRB block. HRB 70 block has a recorded overshoot up to 0.7N while the sensitivity coefficient is ~0.08 HRB/N = -0.056HRB HRB 95 block has a recorded overshoot up to 3.5N while the sensitivity coefficient is ~0.05 HRB/N = -0.175HRB. This shows the overshoot from the force could be the cause of the error for the 95 HRB block and also closes the difference between IMGC. It is less significant on the softer 70HRB block. These results seem to show little major effect on the readings are caused by the velocity difference, and that the NPL machine is better with slower velocities. Indenter comparison As mentioned the indenter is the other major cause for error. Characterisation of the NPL and IMGC indenter is displayed in Table 12. Table 12. NPL and IMGC characteristics. IMGC Indenter Cone angle: 119.95° (max deviation 0.07°) Tilt angle: 0.04° Generatrix planarity: 0.2 um Spherical tip radius: 200.2 um (SD 2.1 um) NPL Indenter Cone angle: 119.725° (SD=0.025) Tilt angle 0.130 (SD= 0.009) Generatrix planarity: 0.2 um There is a difference of 0.225° between the NPL and IMGC indenters cone angles, this corresponds to a 0.32-0.1 HRC difference for 20HRC-65HRC respectively, based on the sensitivity coefficients. These errors are of a similar magnitude to the errors measured in the comparison (0.34HRC for the 20 HRC block). Showing the main errors is the indenter.

Findings/Conclusions

It has been shown from the data, from this inter-comparison, that the NPL and the IMGC hardness machines compare well. When using the same indenter, readings on all the scales used were very close. It was identified that the indenters were a major cause of errors between the two labs. Results with different indenters showed much larger errors. Sensitivity coefficients can correct a large proportion of these errors, when used to adjust for the differences in the two indenters. The difference in the velocity for the last 20% of the application of F, within the NPL profile and that of IMGC’s, also has some effect on the error making it worse. Correcting this in theory should improve the differences between our results. But as shown the NPL machine struggles with these speeds and overshoots negating the positive effect of comparable speeds within the profiles. This makes it unrealistic to mirror the IMGC profile in this way. The general difference between NPL and IMGC showed NPL to give lower hardness values. This actually shows that NPL calibrates the blocks closer to the values obtained by UKAS Labs. Work done in a comparison with these labs shows NPL to compare well but giving slightly harder readings. Therefore there should be no real problems with the UK labs when NPL takes over the national standard from IMGC, due to this good comparability.