<PAGE>   1
                                                                    Exhibit 10.8


                      FOURTH AMENDMENT TO REVOLVING CREDIT
                     AND LETTER OF CREDIT ISSUANCE AGREEMENT

                  This FOURTH AMENDMENT TO REVOLVING CREDIT AND LETTER OF CREDIT
ISSUANCE AGREEMENT is made as of this 8th day of September, 1999 (this "FOURTH
AMENDMENT") and entered into by and among THE CARBIDE/GRAPHITE GROUP, INC., a
corporation organized and existing under the laws of the State of Delaware (the
"BORROWER"), the financial institutions party thereto as lenders (collectively
referred to herein as the "LENDERS"), PNC BANK, NATIONAL ASSOCIATION, in its
capacity as the issuer of letters of credit (in such capacity, the "L/C ISSUER")
and PNC BANK, NATIONAL ASSOCIATION, in its capacity as agent for the Lenders and
the L/C Issuer (in such capacity, the "AGENT") and further amends that certain
Revolving Credit and Letter of Credit Issuance Agreement dated as of September
25, 1997, as previously amended by that certain First Amendment to Revolving
Credit and Letter of Credit Issuance Agreement dated as of October 28, 1997 (the
"FIRST AMENDMENT"), the Second Amendment to Revolving Credit Agreement and
Waiver dated as of April 30, 1998 (the "SECOND AMENDMENT") and the Third
Amendment to Revolving Credit and Letter of Credit Issuance Agreement and
Amendment to Revolving Credit Notes dated as of April 30, 1999 (the "THIRD
AMENDMENT") (the Revolving Credit and Letter of Credit Issuance Agreement, as
amended by the First Amendment, the Second Amendment and the Third Amendment, is
hereinafter referred to as the "ORIGINAL CREDIT AGREEMENT").


                                   WITNESSETH


                  WHEREAS, as a result of the Investigation the Borrower has
recorded, without admitting or denying either culpability or liability, a charge
of Thirty Eight Million Dollars ($38,000,000) for its Fiscal Quarter ending
April 30, 1998 for liabilities which may be incurred as a result of the
Investigation and will record, without admitting or denying either culpability
or liability, a charge of Seven Million Dollars ($7,000,000) for its Fiscal
Quarter ending July 31, 1999; and

                  WHEREAS, the Borrower has requested certain amendments to the
terms of the Original Credit Agreement to accommodate the Investigation and the
Special Reserve (as defined below); and

                  WHEREAS, the Agent, the Lenders and the L/C Issuer have agreed
to make such amendments upon the terms and conditions set forth herein.

                  NOW THEREFORE, in consideration of the foregoing premises, the
mutual covenants and agreements contained herein and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, and
with the intent to be legally bound hereby, the parties hereto agree as follows:



<PAGE>   2


                                    ARTICLE I
                     AMENDMENTS TO ORIGINAL CREDIT AGREEMENT

         SECTION 1.01 AMENDMENTS TO SECTION 1.01 OF THE ORIGINAL CREDIT
AGREEMENT. The following defined terms and the definitions therefor are hereby
added to Section 1.01 of the Original Credit Agreement and inserted in correct
alphabetical order:

                  Fourth Amendment shall mean the Fourth Amendment to Revolving
         Credit and Letter of Credit Issuance Agreement dated as of the Fourth
         Amendment Effective Date.

                  Fourth Amendment Effective Date shall mean September 8, 1999.

                  Fourth Amendment Fee shall mean a fee equal to five (5) basis
         points of each Lender's Revolving Credit Commitment which shall be
         payable by the Borrower to each Lender who approves the Fourth
         Amendment on or before the Fourth Amendment Effective Date.

                  Initial Special Reserve shall mean the charge of Thirty-Eight
         Million Dollars ($38,000,000) recorded by the Borrower for its Fiscal
         Quarter ending April 30, 1998 for potential liabilities which may be
         incurred as a result of the Investigation.

                  Supplemental Special Reserve shall mean the charge of Seven
         Million Dollars ($7,000,000) recorded by the Borrower for its Fiscal
         Quarter ending July 31, 1999 for potential liabilities which may be
         incurred as a result of the Investigation.

         SECTION 1.02 MODIFICATION OF EXISTING DEFINED TERM. The following
defined terms in Section 1.01 of the Original Credit Agreement are amended and
restated to read as follows:

                  EBITDA shall mean, for any period, the consolidated net income
         (or net loss) of the Borrower for such period as determined in
         accordance with GAAP, plus (a) the sum of (i) depreciation expense,
         (ii) amortization expense, (iii) Interest Expense, (iv) total income
         tax expense, (v) extraordinary or unusual losses (including (a) the
         Initial Special Reserve and the Supplemental Special Reserve and (b)
         after tax losses on sales of assets outside of the ordinary course of
         business and not otherwise included in GAAP extraordinary or unusual
         losses), (vi) other non-cash charges, and (vii) the net loss of any
         Person that is accounted for by the equity method of accounting, except
         to the extent of the amount of dividends or distributions paid to the
         Borrower, less (b) the sum of (i) extraordinary or unusual gains
         (including after tax gains on sales of assets outside of the ordinary
         course of business and not otherwise included in GAAP extraordinary or
         unusual gains), (ii) other non-cash credits, and (iii) the net income
         of any Person that is accounted for by the equity method of accounting,



                                      -2-
<PAGE>   3


         except to the extent of the amount of dividends or distributions paid
         to the Borrower.

                  Special Reserve shall mean for all periods prior to April 30,
         1999 the Initial Special Reserve and for all periods after May 1, 1999
         collectively the Initial Special Reserve and the Supplemental Special
         Reserve.

         SECTION 1.03 NO OTHER AMENDMENTS. Except as specifically set forth
therein, the amendments to the Original Credit Agreement set forth in Sections
1.01 through 1.02 above do not either implicitly or explicitly alter, waive or
amend, the provisions of the Original Credit Agreement. The amendments set forth
in Sections 1.01 through 1.02 hereof do not waive, now or in the future,
compliance with any other covenant, term or condition to be performed or
complied with nor do they impair any rights or remedies of the Lenders and the
Agents under the Original Credit Agreement with respect to any such violation.



                                   ARTICLE II
                     BORROWER'S SUPPLEMENTAL REPRESENTATIONS

         SECTION 2.01. INCORPORATION BY REFERENCE. As an inducement to the
Agent, the Lenders, and the L/C Issuer to enter into this Fourth Amendment, the
Borrower hereby repeats herein for the benefit of the Agent, the Lenders, and
the L/C Issuer the representations and warranties made by the Borrower in
Article IV of the Original Credit Agreement, as amended hereby, except that for
purposes hereof such representations and warranties shall be deemed to extend to
and cover this Fourth Amendment.

                                   ARTICLE III
                              CONDITIONS PRECEDENT

         SECTION 3.01 CONDITIONS PRECEDENT. Each of the following shall be a
condition precedent to the effectiveness of this Fourth Amendment:

                  (i) The Agent shall have received, on or before the Fourth
Amendment Effective Date, the following items, each, unless otherwise indicated,
dated on or before the Fourth Amendment Effective Date and in form and substance
satisfactory to the Agent and its special counsel, Tucker Arensberg, P.C.:

                           (A) A duly executed counterpart original of this
Fourth Amendment executed by the Required
Lenders, the Agent and the Borrower;

                           (B) A certificate from the Secretary of the Borrower
certifying that the Articles of Incorporation and Bylaws of the Borrower
previously delivered to the Agent are true, complete, and correct.



                                      -3-
<PAGE>   4


                           (C) Payment of the Fourth Amendment Fee to the Agent
for the benefit of the Lenders;

                           (D) Such other instruments, documents and opinions of
counsel as the Agent shall reasonably require, all of which shall be
satisfactory in form and content to the Agent and its special counsel, Tucker
Arensberg, P.C.

                  (ii) The following statements shall be true and correct on the
Fourth Amendment Effective Date and the Agent shall have received a certificate
signed by an Authorized Officer of the Borrower, dated the Fourth Amendment
Effective Date, stating that:

                           (A) the representations and warranties made pursuant
to Section 2.01 of this Fourth Amendment and in the other Loan Documents, as
amended hereby, are true and correct on and as of the Fourth Amendment Effective
Date as though made on and as of such date;

                           (B) no petition by or against the Borrower has at any
time been filed under the United States Bankruptcy Code or under any similar
act;

                           (C) Taking into account the amendments set forth in
this Fourth Amendment, no Event of Default or event which with the giving of
notice, the passage of time or both would become an Event of Default has
occurred and is continuing, or would result from the execution of or performance
under this Fourth Amendment;

                           (D) Taking into account the amendments set forth in
this Fourth Amendment, no Material Adverse Change has occurred which has not
been disclosed to the Agent and the Lenders; and

                           (E) Taking into account the amendments set forth in
this Fourth Amendment, the Borrower has in all material respects performed all
agreements, covenants and conditions required to be performed on or prior to the
date hereof under the Original Credit Agreement and the other Loan Documents.



                                   ARTICLE IV
                               GENERAL PROVISIONS

         SECTION 4.01 RATIFICATION OF TERMS. Except as expressly amended by this
Fourth Amendment, the Original Credit Agreement and each and every
representation, warranty, covenant, term and condition contained therein is
specifically ratified and confirmed. The Borrower hereby confirms that any
collateral for the Lender Obligations, including but not limited to
encumbrances, Liens, security interests, mortgages and pledges granted by the
Borrower or third parties, shall continue unimpaired and in full force and
effect. THE BORROWER EXPRESSLY RATIFIES AND CONFIRMS THE WAIVER OF JURY TRIAL
PROVISION CONTAINED IN THE ORIGINAL CREDIT AGREEMENT AND THE OTHER LOAN
DOCUMENTS.



                                      -4-
<PAGE>   5


         SECTION 4.02 REFERENCES. All notices, communications, agreements,
certificates, documents or other instruments executed and delivered after the
execution and delivery of this Fourth Amendment in connection with the Original
Credit Agreement, any of the other Loan Documents or the transactions
contemplated thereby may refer to the Original Credit Agreement without making
specific reference to this Fourth Amendment, but nevertheless all such
references shall include this Fourth Amendment unless the context requires
otherwise. From and after the Amendment Effective Date, all references in the
Original Credit Agreement and each of the other Loan Documents to the
"Agreement" shall be deemed to be references to the Original Credit Agreement as
amended hereby.

         SECTION 4.03 INCORPORATION INTO ORIGINAL CREDIT AGREEMENT. This Fourth
Amendment is deemed incorporated into the Original Credit Agreement. To the
extent that any term or provision of this Fourth Amendment is or may be deemed
expressly inconsistent with any term or provision of the Original Credit
Agreement, the terms and provisions hereof shall control.

         SECTION 4.04 COUNTERPARTS. This Fourth Amendment may be executed in
different counterparts, each of which when executed by the Borrower and the
Agent, the Lenders, and the L/C Issuer shall be regarded as an original, and all
such counterparts shall constitute one Fourth Amendment.

         SECTION 4.05 CAPITALIZED TERMS. Except for proper nouns and as
otherwise defined herein, capitalized terms used herein as defined terms shall
have the same meanings herein as are ascribed to them in the Original Credit
Agreement, as amended hereby.

         SECTION 4.06 TAXES. The Borrower shall pay any and all stamp and other
taxes and fees payable or determined to be payable in connection with the
execution, delivery, filing and recording of this Fourth Amendment and such
other documents and instruments as are delivered in connection herewith and
agrees to save the Agent, the Lenders, and the L/C Issuer harmless from and
against any and all liabilities with respect to or resulting from any delay in
paying or omission to pay such taxes and fees.

         SECTION 4.04 COSTS AND EXPENSES. The Borrower will pay all costs and
expenses of the Agent (including, without limitation, the reasonable fees and
the disbursements of the Agent's special counsel, Tucker Arensberg, P.C.) in
connection with the preparation, execution and delivery of this Fourth Amendment
and the other documents, instruments and certificates delivered in connection
herewith.

         SECTION 4.08 GOVERNING LAW. THIS FOURTH AMENDMENT AND THE RIGHTS AND
OBLIGATIONS HEREUNDER SHALL BE CONSTRUED IN ACCORDANCE WITH AND GOVERNED BY THE
LAWS OF THE COMMONWEALTH OF PENNSYLVANIA WITHOUT REGARD TO THE PROVISIONS
THEREOF REGARDING CONFLICTS OF LAW.

         SECTION 4.09 HEADINGS. The headings of the sections in this Fourth
Amendment are for purposes of reference only and shall not be deemed to be a
part hereof.




                                      -5-
<PAGE>   6


                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]












                                      -6-
<PAGE>   7


                  IN WITNESS WHEREOF, the parties hereto, with the intent to be
legally bound hereby, have caused this Fourth Amendment to Revolving Credit and
Letter of Credit Issuance Agreement to be duly executed by their respective
proper and duly authorized officers as a document under seal, as of the day and
year first above written.

ATTEST:                                       BORROWER:

                                              THE CARBIDE/GRAPHITE GROUP,
                                              INC., a Delaware corporation



_________________________                     By__________________________(SEAL)
Name:                                         Name:
Title:                                        Title:

                                              AGENT AND L/C ISSUER:

                                              PNC BANK, NATIONAL ASSOCIATION


                                              By__________________________(SEAL)
                                              Name:
                                              Title:


                                              LENDERS:

REVOLVING CREDIT                              PNC BANK, NATIONAL ASSOCIATION
COMMITMENT:       $26,000,000.00

RATABLE SHARE:    17.33%                      By__________________________(SEAL)
                                              Name:
                                              Title:


                    [SIGNATURES CONTINUED ON FOLLOWING PAGE]





                                      -7-
<PAGE>   8


REVOLVING CREDIT                              NATIONAL CITY BANK OF
COMMITMENT:       $18,000,000.00              PENNSYLVANIA


RATABLE SHARE:    12%                         By__________________________(SEAL)
                                              Name:
                                              Title:


REVOLVING CREDIT                              THE FIRST NATIONAL BANK OF
COMMITMENT:       $13,000,000.00              CHICAGO

RATABLE SHARE:    8.66%                       By__________________________(SEAL)
                                              Name:
                                              Title:


REVOLVING CREDIT                              FIRST UNION NATIONAL BANK
COMMITMENT:       $18,000,000.00

RATABLE SHARE:    12%                         By__________________________(SEAL)
                                              Name:
                                              Title:

REVOLVING CREDIT                              KEYBANK, NATIONAL ASSOCIATION
COMMITMENT:       $13,000,000.00

RATABLE SHARE:    8.66%                       By__________________________(SEAL)
                                              Name:
                                              Title:


REVOLVING CREDIT                              STANDARD CHARTERED BANK
COMMITMENT:       $13,000,000.00

RATABLE SHARE:    8.66%                       By__________________________(SEAL)
                                              Name:
                                              Title:


                    [SIGNATURES CONTINUED ON FOLLOWING PAGE]




                                      -8-
<PAGE>   9


REVOLVING CREDIT                              MELLON BANK, N.A.
COMMITMENT:       $13,000,000.00

RATABLE SHARE:    8.66%                       By__________________________(SEAL)
                                              Name:
                                              Title:


REVOLVING CREDIT                              BANK OF AMERICA, N.A.
COMMITMENT:       $18,000,000.00

RATABLE SHARE:    12%                         By__________________________(SEAL)
                                              Name:
                                              Title:


REVOLVING CREDIT                              THE CHASE MANHATTAN BANK
COMMITMENT:       $18,000,000.00

RATABLE SHARE:    12%                         By__________________________(SEAL)
                                              Name:
                                              Title:






                                      -9-