The Prince of Wales has unveiled a plaque at Tata Steel's Port Talbot plant on Friday to mark a £185m project to rebuild furnace number four.
It has taken four months to complete, as part of a £240m investment in the site, including more than £50m on upgrading the steel-making shop.
The prince then visited Swansea to see the market and the house where Dylan Thomas was born.
The tour ended with a traditional Welsh carol service in Carmarthen.
Tata Steel announced last month that it planned to restart the furnace in the first quarter of 2013.
According to Tata, the operation has been the largest industrial construction project of 2012 in the UK.
The furnace will be restarted over the Christmas period, before running up to full operation in early 2013.
At full capacity it will provide an extra half-million tonnes of liquid iron at the site.
The prince arrived at the steelworks on board the royal train.
He previously visited the site in 2002 to reopen furnace number five, rebuilt after an explosion which killed three workers and injured a dozen others in 2001.
"It is always an enormous pleasure for me to come back here and visit this incredible place," the prince told a group of senior Tata Steel workers.
"I know just what an extraordinary effort was required to put this project together.
"Having heard a little about it, I understand that it will become one of the most sophisticated blast furnaces in Europe."
When the prince arrived he was ushered into a specially erected marquee and was shown a video about the lightweight super-strength steel which could transform safety for British soldiers.
"It will give our troops a level of protection they have not had before and which nobody in the world offers. It is unique to the UK," a Tata spokesman said.
He said the anti-ballstic steel, which has a lattice form, could be used to armour-plate troop carriers and tanks.
As the Prince of Wales was taken around the marquee, children from Eastern Primary School in Port Talbot performed a series of festive songs.
The good cheer generated by the royal visit comes despite Tata Steel's recent announcement of hundreds of job losses.
It is the prince's second visit to Wales in as many weeks, after he met victims of flooding in north Wales at St Asaph.
After visiting the steelworks, the prince travelled to Swansea, to visit the city's indoor market and the birthplace of Dylan Thomas at Cwmdonkin Drive.
The prince's visit to the poet's first home comes as the city and Wales prepares to mark the centenary of his birth.
He toured the house, which has been restored to how it would have looked when Thomas lived there from 1914.
Inside the living room, the prince was treated to tea and Welsh cakes and was joined by the poet's granddaughter Hannah Ellis.
"This is a living house not a museum and so visitors are encouraged to touch, sit and explore the rooms which are decorated and furnished as they would have been when the Thomas family moved in," said Geoff and Anne Haden, who restored the house.
Finally, Prince Charles joined a traditional Welsh carol service in Carmarthen, at the town's University of Wales Trinity Saint David campus.
In the service - known as the plygain - members of the congregation are invited to perform carols, often handed down through families over generations, and almost always sung unaccompanied by music.