Charpy Machining Qualification.docx

NIST Charpy V-Notch Standard Reference Materials Machining Qualification Program Policy

Background:

The purpose of this qualification procedure is to identify and qualify machine shops and producers (“Vendors”) capable of machining and supplying Charpy V-Notch Specimens according to NIST specifications. Qualified Vendors will be placed on the Qualified Manufacturers List (QML).

Vendors will be compensated up to $6000 if they are able to meet identified performance requirements. Compensation is not dependent on successful certification, only on the required performance metrics demonstrating significant effort expended in the process.

NIST has Vendors produce reference-grade Charpy Impact Specimens in batches. The process has three major steps:

1) Rough machining, where the steel bar is cut to length and machined into "blanks." The blanks are unnotched small bars and have over-sized dimensions suitable for heat treating and subsequent machining. Blanking eliminates surface defects that can induce quench cracks during processing, and the oversized dimensions of the blanks allow for re-processing if necessary.

2) Heat treating the blanks, so that the final machined specimens meet the NIST acceptance criteria specified in Appendix C.

3) Final machining of heat-treated specimens to final dimensions, as indicated in Appendix A.

For full qualification, a Vendor must demonstrate the ability to rough machine blanks of NIST steel, heat treat the blanks, and finally machine the specimens, so that all dimensional and performance criteria are met.

Application:

Test specimen lots for the Charpy Machine Verification Program will be accepted throughout the calendar year. The Vendor must be on the QML list prior to the issuance of the procurement action.

Successful completion of the Machining Qualification Program described herein places the applicant on the Qualified Manufacturers List (QML). Potential vendors are highly encouraged to respond to the upcoming Request for Quotation (RFQ) on www.beta.SAM.gov. Upon completion of the RFQ process and awards, the authorized QML Vendors are given a fair opportunity to machine Charpy specimens Standard Reference Materials (SRMs) through a competition called delivery orders among multiple-award indefinite-delivery/indefinite-quantity (IDIQ) awardees.

Qualification does not guarantee a contract; however, all IDIQ awardees are offered a minimum guarantee in the base year. Those Vendors that fail qualification will be promptly notified and furnished specific reasons why qualification was not attained.

The Charpy Coordinator will maintain the QML. Vendors shall not be charged a fee for submitting test specimens to be evaluated, nor shall Vendors be reimbursed for any test specimens submitted.

Inclusion on this list does not constitute an endorsement of any product, source, or manufacturer by the National Institute of Standards and Technology.

Period of Qualification:

Qualification is valid for five years, starting with successful approval of the initial qualification procedure. Acceptance of at least one lot of Charpy specimens per calendar year renews the Vendor's qualification for another five years. Thus, the production of at least five acceptable lots of Charpy specimens within five years will result in remaining on the QML for the next five years.

If a Vendor fails to perform adequately during the five years (less than five acceptable lots produced) or if for some other reason the Vendor is deemed unable to perform adequately, the Vendor shall be promptly notified, removed from the QML, furnished specific reasons for removal, and asked if they wish to requalify after taking corrective actions.

Qualification Requirements

Documentation Required:

The Vendor shall supply a company description. This information will be used to evaluate the technical competence of the Vendor, and assist NIST in answering any technical questions that may arise. All information agreed upon as proprietary shall be held in the strictest confidence. NIST reserves the right to visit the Vendor's facilities to audit the procedures and equipment.

Specimen Dimensions and Marking:

All specimens submitted shall conform to the dimensional specifications listed in Appendix A. The specimen identification marking scheme is outlined in Appendix B.

Qualification step 1: Specimen Machining The Vendor shall first supply a preliminary lot consisting of 50 Charpy specimens from any steel supplied by the Vendor, in accordance with the dimensions and tolerances specified in Appendix A and marked/identified in accordance with Appendix B. The aim of step 1 is to confirm the Vendor's ability to machine samples that meet NIST requirements and fulfill the specified tolerances. The preliminary lot shall be delivered to NIST within 30 days of the start of the qualification process.

Qualification step 2: Full Qualification Following successful completion of step 1, the Vendor can fully qualify for the production of NIST Charpy SRMs by completing step 2. In order to do this, the Vendor shall heat treat and machine a small specimen lot. The lot shall consist of 300 specimens machined and heat-treated from steel supplied by NIST to the Vendor free of charge. The Vendor can choose whether to produce a lot of low-energy, high-energy, or super-high energy specimens. In the event that the first 300-specimen lot is not acceptable, the Vendor may submit one additional 300-specimen lot. The 300-specimen lot shall be delivered to NIST within 30 days of the acceptance of the preliminary lot (step 1). The additional 300-specimen lot, if applicable, shall be delivered to NIST within 30 days of the rejection of the previous 300-specimen lot.

Mail all qualification lots and documentation to:

National Institute of Standards and Technology Charpy Coordinator, Allen Eckhardt Division 647 325 Broadway Boulder, Colorado 80305-3328 Questions may be directed to:

Enrico Lucon, Charpy Project Leader

Appendix A

The Vendor shall manufacture Charpy V-notch specimens to the following dimensions and tolerances.

Specimen Dimensions and Tolerances:

1. The material shall be cut and ground to the finished length of 55 mm (2.165 in. ).

2. One end of each specimen shall be stamped with the lot number and the serial number, and the other end shall be stamped with the letters: NIST. See Appendix B: Marking Test Lots, for details.

3. The Vendor shall determine the exact heat treatment needed to produce the required energy level.

4. The specimen cross-section dimensions shall be 10 mm ± 0.03 mm (0.394 ± 0.001 in.) ground square; any two adjacent sides shall be square to 90° ± 9'.

5. The surface roughness shall be 1.6 µm (47 µin.) or less on the notched surface and opposite surface, and 3.2 µm (100 µin.) or less on the other two longitudinal surfaces, plus both ends.

6. Notch and Side-Groove Specifications:

a. The notch shall be centered at 27.5 mm ± 0.20 mm (1.083 in. ± 0.008 in.).

b. The ligament size below the notch shall be 8.0 mm ± 0.025 mm (0.315 in. ± 0.001 in.).

c. The notch shall be perpendicular to the longitudinal axis of the specimen to 90° ± 9'.

d. The notch angle shall be 45° ± 1°, with a root radius of 0.25 mm ± 0.025 mm (0.010 in. ± 0.001 in.).

e. If side-grooves are required (super-high-energy specimens), specimens shall be side-grooved for a total reduction of 14 % of the original thickness, corresponding to 0.8 mm ± 0.05 mm (0.0315 in. ± 0.002 in.) per side (unless otherwise specified). Side-grooves shall have an included angle ≤ 90° and a root radius of 0.25 mm ± 0.05 mm (0.01 in. ± 0.002 in.).

f. Specimen notches and side-grooves shall be ground on a surface grinder. Machining with a fly cutter, multitooth cutter, or broach is NOT acceptable. In general, the use of an electrical discharge machining (EDM) process is not allowed, unless the notch profile is found acceptable based on a visual inspection of multiple specimens by the Charpy Coordinator. Should the EDM notch profile be judged unsatistifactory, grinding shall be used instead. The use of EDM for side-groove preparation is allowed. Overheating or cold working the material adjacent to the notch or side-groove shall be avoided. For grinding, the next to last cut shall be more than 0.25 mm (0.010 in.) and less than 0.38 mm (0.015 in.). Appropriate machining oil shall be used during all grinding or machining operations. Notches and side-grooves shall be deburred.

As already stated above, permissible variations (tolerances) shall be as follows:

Notch length to edge: 90o 9'

Adjacent sides shall be at: 90o 9'

Cross-section dimensions: 10 mm 0.03 mm (0.394 in. 0.001 in.)

Length of specimen (L): 55 mm + 0.00 _ 0.30 mm (2.165 in. + 0.00 in. –0.012 in.)

Centering of the notch (L/2): 27.5 mm 0.20 mm (1.083 in. 0.008 in.)

Opening angle of notch: 45o 1o

Radius of notch 0.25 mm 0.025 mm (0.010 in. 0.001 in.)

Ligament size under the notch 8.0 mm 0.025 mm (0.315 in. 0.001 in.)

Side-groove depth 0.8 mm ± 0.05 mm (0.0315 in. ± 0.002 in.)

Side-groove angle ≤ 90°

Side-groove root radius 0.25 mm ± 0.05 mm (0.01 in. ± 0.002 in.)

Finish requirements: The surface roughness shall be 1.6 µm (47 µin.) or less on the notched surface and opposite surface, and 3.2 µm (100 µin.) or less on the two longitudinal surfaces, plus both ends.

Appendix B Marking Specimen Lots

1. The Vendor's name and the lot id shall be placed on, or adjacent to, all exterior mailing or shipping labels of test lot shipments.

2. Individual specimens shall be stamped or etched on one end with one of the following lot ids:

where 'NNN' is the lot number, while "LL," "HH," and "SH" correspond to low-energy, high-energy, and super-high energy lots, respectively. Lot numbers will be assigned by the Charpy Coordinator.

3. The specimen serial number shall be stamped or etched below the lot number. Serial numbers shall begin with the number 1 and end with the last number produced within the lot.

4. The opposite end shall be stamped or etched with the letters: NIST.

Appendix C Acceptance Criteria

1. For all specimens, dimensions shall be within the tolerances specified in Appendix A.

2. The hardness of each specimen shall be within ± 1.5 HRC of the average of the 300-specimen lot.

3. For low-energy specimens, the average hardness shall not be less than HRC 44.

4. The acceptable ranges for the average energies of qualification lots are as follows:

a. Low-energy specimens shall produce an average energy between 13 J and 20 J (9.6 ft-lbf to 14.8 ft-lbf) at 21 °C ± 1 °C.

b. High-energy specimens shall produce an average energy between 85 J and 140 J (62.7 ft-lbf to103.3 ft-lbf) at 21 °C ± 1 °C.

c. Super-High energy specimens shall produce an average energy between 175 J and 240 J (129.1 ft-lbf to 177 ft-lbf) at 21 °C ± 1 °C.

Any lot whose average energy lies outside the ranges above shall be subject to final approval by the Charpy Program coordinator and the Charpy Project Leader.

5. Specimens at all energy levels should meet the requirements below:

a. The sample size of the lot shall not exceed 5. The sample size (nSS) is calculated as follows:

where E is equal to 1.4 J or 5 % of the average energy for the 3 NIST reference machines (whichever is greater), and Sp is the pooled standard deviation of the 3 reference machines. The standard deviation (s) and the pooled standard deviation (Sp) are calculated as follows:

where s1, s2, and s3 are the standard deviations of the three reference machines.

b. The number of outliers shall not exceed 10 % of the number of specimens tested in the 300-specimen lot. The outlier analysis is performed using box-and-whiskers plots to provide a graphical summary of the data and identify outliers. Outliers are defined as values that are lower than the first quartile or higher than the third quartile by more than 1.5 times the absolute difference between the first and third quartiles. An alternative outlier analysis can be performed by means of Grubbs' test (extreme studentized deviate) with = 0.05. An outlier will not be removed from analysis unless it shows physical evidence of jamming, material flaws, or other reason for atypical behavior.

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