The men were found collapsed on stairs, the HSE said
A firm has been ordered to pay more than £700,000 following the deaths of two workers five years ago.
Richard Clarkson, 29, and Stuart Jordan, 50, who worked for Bodycote HIP Ltd at a Hereford metal refining plant, died in 2004 after an argon gas leak.
The multinational firm was fined £533,000 and ordered to pay costs of £200,000 after admitting breaching the Health and Safety at Work Act.
The court heard the gas leaked from a large pressure vessel in a pit.
The Health and Safety Executive (HSE) brought the case against Bodycote HIP of Macclesfield, Cheshire.
It told Worcester Crown Court that Mr Jordan, a works manager, and Mr Clarkson, a maintenance engineer, were found collapsed on the stairs leading to the pit at the College Road site on 14 June 2004.
The HSE said the pit's oxygen alarm system was switched off on the day and the ventilation system, which it said could have saved the men's lives, was not running.
There was no evidence Mr Jordan had been given any appropriate safety training, the HSE told the court.
The HSE also said the firm had failed to learn the lessons of a similar double fatality at a plant in California three years earlier.
The company told the court in its defence that complacency had developed but there was not a wholesale disregard for health and safety.
The firm added that the health and safety deficiencies had since been remedied.
After the case, HSE inspector Luke Messenger said: "Both these tragic deaths were not only regrettable but also entirely preventable".
He added: "Confined spaces can be found in a wide range of workplaces and these deaths should serve as a reminder to all industries of the dangers of this type of work."
In January 2008, an inquest verdict of unlawful killing on the two men was overturned by the High Court.