Union officials say steelworkers could gain up to $10 million in back pay as a result of an arbitration board's ruling against USX Corp.

The board's ruling, released by the United Steelworkers of America on Monday, compels USX to reimburse workers at overtime rates in instances - totaling as many as 1,100 - where work was lost because of outside hiring that violated the steelworkers' contract, union chiefs said."It's an enormous victory for the union that shows that what we've said about contracting out is true," Steelworkers union president Lynn Williams told reporters.

The arbitration board examined 24 grievances at the company's Fairless Works outside Philadelphia. The union and the company selected those cases because they were said to represent some 1,100 pending complaints by workers.

The board held that back pay was owed in all but five of the 24 cases. No award was granted in three, and the remaining two were returned to the parties for more information.

The union studied the back pay awards and found one to be worth about $700,000.

"Another of the 24 test cases involved 900-1,000 hours and several involved 200-300 hours. If these are typical of the 1,100 pending cases, the company's liability should easily reach $8 million to $10 million," union leaders said in a memorandum to members.

The document urged workers to "guard against wheeling and dealing aimed at selling your jobs to outside contractors or buying the right to do so." The issue of outside hiring had been central to a six-month walkout by USX workers in 1987.

In his decision, arbitration board chairman Alfred Dybeck also said that after one year, a "cease and desist" order will be in effect barring outside hiring for specified types of work.