El Segundo, Calif.-based Huber, owner of about 8.2% of A.M. Castle, joins an affiliate of Platinum Equity LLC in pressuring the company. A.M. Castle enacted a poison pill after Ryerson Inc., which is controlled by Platinum, in August disclosed it had bought a 6.1% stake in Castle and said it wished to engage in discussions about strategic alternatives with the company.

Ryerson is a distributor of metal products.

Huber in a regulatory filing late Tuesday said it has been a shareholder of Oak Brook, Ill.-based A.M. Castle since 2007, but noted that the company's shares have lost about half of their value since the firm's initial purchase. The investor said that it was disappointed with A.M. Castle's November 2011 purchase of Tube Supply Inc. for $165 million, a deal that Huber describes as "made at the top of the market" for oil and gas fracking equipment.

The deal was also financed at what Huber calls "an exorbitant 13.7%" at a time of record-low interest rates.

The investor was also angered that A.M. Castle failed to grant Huber a waiver to its poison pill, which would have allowed it to continue to build its stake in the company. Huber said it had offered to enter into a one-year standstill agreement in return for the waiver, but was rejected.

Analysts have said that A.M. Castle's exposure to the commercial aerospace industry would make it an attractive target for larger component makers. The company, which has just over $1 billion in annual sales, has been mentioned as a potential target for acquisitive Reliance Steel & Aluminum Co., which said last month it would consider larger purchases.